Remove trailing ':' from racer execution

When AWK processes the racer response, it adds a trailing ':' that
kakoune completion expects to lead to a row having an 'a|b|c' shape,
but it's empty instead. Removing the extra ':' allows the racer
complation to actually work.

rc/extra/racer.kak

@@ -45,7 +45,7 @@ define-command racer-complete -docstring "Complete the current selection with ra
                 }'
             )
             printf %s\\n "evaluate-commands -client '${kak_client}' %{
-                set-option buffer=${kak_bufname} racer_completions %@${compl}@
+                set-option buffer=${kak_bufname} racer_completions %@${compl: : -1}@
             }" | kak -p ${kak_session}
             rm -r ${dir}
         ) > /dev/null 2>&1 < /dev/null &