rc modeline: fix error trying to write readonly global variable
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

mawww/kakoune,jkonecny12/kakoune,mawww/kakoune,mawww/kakoune,jkonecny12/kakoune,mawww/kakoune,jkonecny12/kakoune,jkonecny12/kakoune