Remove flake8.kak, deprecated by lint.kak

Builtin support for `flake8` can be obtained using the following:
```
set window lintcmd 'flake8 --filename=* --format="%(path)s:%(row)d:%(col)d: error: %(text)s"'
```

rc/extra/flake8.kak

@@ -1,87 +0,0 @@
-decl str flake8_options
-decl -hidden str flake8_tmp_dir
-decl -hidden line-flags flake8_flags
-decl -hidden str flake8_errors
-
-def flake8-lint -docstring "Lint the contents of the current buffer" %{
-    %sh{
-        dir=$(mktemp -d -t kak-flake8.XXXXXXXX)
-        mkfifo ${dir}/fifo
-        echo "set buffer flake8_tmp_dir ${dir}"
-        echo "eval -no-hooks write ${dir}/buf"
-    }
-
-    # end the previous %sh{} so that its output gets interpreted by kakoune
-    # before launching the following as a background task.
-    %sh{
-        dir=${kak_opt_flake8_tmp_dir}
-        echo "eval -draft %{
-            edit! -fifo ${dir}/fifo *flake8-output*
-            set buffer filetype make
-            set buffer _make_current_error_line 0
-            hook -group fifo buffer BufCloseFifo .* %{
-                nop %sh{ rm -r ${dir} }
-                rmhooks buffer fifo
-            }
-        }"
-        # this runs in a detached shell, asynchronously, so that kakoune does
-        # not hang while flake8 is running.
-        (
-            flake8 --ignore=${kak_opt_flake8_options} - < ${dir}/buf > ${dir}/stderr
-            flags=$(cat ${dir}/stderr | sed -rne "
-                /^stdin:[0-9]+:[0-9]+:? (W|F|C|N).*/ { s/^stdin:([0-9]+):.*/\1|{yellow}█/; p }
-                /^stdin:[0-9]+:[0-9]+:? E.*/ { s/^stdin:([0-9]+):.*/\1|{red}█/; p }
-            " | paste -s -d ':')
-            errors=$(cat ${dir}/stderr | sed -rne "
-                /^stdin:[0-9]+:[0-9]+:?.*/ { s/^stdin:([0-9]+):([0-9]+:)? (.*) /\1,\3/; s/'/\\\\'/g; p }
-            " | sort -n)
-
-            sed -e "s|<stdin>|${kak_bufname}|g" < ${dir}/stderr > ${dir}/fifo
-            echo "set 'buffer=${kak_buffile}' flake8_flags %{${kak_timestamp}:${flags}}
-                  set 'buffer=${kak_buffile}' flake8_errors '${errors}'" | kak -p ${kak_session}
-        ) > /dev/null 2>&1 < /dev/null &
-    }
-}
-
-def -hidden flake8-show-error-info %{ %sh{
-    desc=$(printf %s\\n "${kak_opt_flake8_errors}" | sed -ne "/^${kak_cursor_line},.*/ { s/^[[:digit:]]\+,//g; s/'/\\\\'/g; p }")
-    if [ -n "$desc" ]; then
-        echo "info -anchor ${kak_cursor_line}.${kak_cursor_column} '${desc}'"
-    fi
-} }
-
-def flake8-enable-diagnostics -docstring "Activate automatic diagnostics of the code" %{
-    addhl flag_lines default flake8_flags
-    hook window -group flake8-diagnostics NormalIdle .* %{ flake8-show-error-info }
-}
-
-def flake8-disable-diagnostics -docstring "Disable automatic diagnostics of the code" %{
-    rmhl hlflags_flake8_flags
-    rmhooks window flake8-diagnostics
-}
-
-def flake8-diagnostics-next -docstring "Jump to the next line that contains an error" %{ %sh{
-    printf "%s\n" "${kak_opt_flake8_errors}" | (
-        line=-1
-        first_line=-1
-        while read line_content; do
-            candidate=${line_content%%,*}
-            if [ -n "$candidate" ]; then
-                if [ $first_line -eq -1 ]; then
-                    first_line=$candidate
-                fi
-                if [ $candidate -gt $kak_cursor_line ]; then
-                    if [ $candidate -lt $line ] || [ $line -eq -1 ]; then
-                        line=$candidate
-                    fi
-                fi
-            fi
-        done
-        if [ $line -eq -1 ]; then
-            line=$first_line
-            echo 'echo -color Error no next flake8 diagnostic'
-        else
-            echo "exec ${line} g"
-        fi
-    )
-}}