rc modeline: fix error trying to write readonly global variable

When running modeline-parse on this file:

	# kakoune: filetype=ledger:indentwidth=4

I get this error from dash (and a similar one from bash):

	sh: 53: readonly: key: is read only

This is because the readonly variable "key" is used elsewhere, both
times as global. Fix this by making both variables local. While
at it, remove an unused variable.

Fixes #4478

rc/detection/modeline.kak

@@ -16,9 +16,8 @@ define-command -hidden modeline-parse-impl %{
 
         # Translate a vim option into the corresponding kakoune one
         translate_opt_vim() {
-            key="$1"
-            value="$2"
-            tr=""
+            local key="$1"
+            local value="$2"
 
             case "${key}" in
                 so|scrolloff)
@@ -60,8 +59,8 @@ define-command -hidden modeline-parse-impl %{
 
         # Pass a few whitelisted options to kakoune directly
         translate_opt_kakoune() {
-            readonly key="$1"
-            readonly value="$2"
+            local readonly key="$1"
+            local readonly value="$2"
 
             case "${key}" in
                 scrolloff|tabstop|indentwidth|autowrap_column|eolformat|filetype|BOM|spell_lang);;