"inline_local_variable" assist shouldn't be applicable if the variable is unused

[to-mas-kral]

[flodiebold]

This should be easy to fix: here we have a list of all references to the variable, so we just need to check whether there are any and return None otherwise.

(And of course add a test using check_assist_not_applicable.)

[to-mas-kral]

Yeah, I'll do that.