gdb_rl_find_completion_word: Remove 'found_quote' local

gdb_rl_find_completion_word: Remove 'found_quote' local

Compiling GDB with current git Clang (future 13) runs into this:

 src/gdb/completer.c:287:18: error: variable 'found_quote' set but not used [-Werror,-Wunused-but-set-variable]
   int scan, end, found_quote, delimiter, pass_next, isbrk;
		  ^

gdb_rl_find_completion_word came to life as a modified (stripped down)
version of readline's internal _rl_find_completion_word function.
When I added it, I don't remember whether I realized that
'found_quote' wasn't really necessary.  Maybe I kept it thinking of
keeping the source code in sync with readline?  I don't recall
anymore.  Since the function is already stripped down compared to the
original, stripping it down some more doesn't hurt.

So fix the issue by removing the unnecessary code.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	* completer.c (RL_QF_SINGLE_QUOTE, RL_QF_DOUBLE_QUOTE)
	(RL_QF_BACKSLASH, RL_QF_OTHER_QUOTE): Delete.
	(gdb_rl_find_completion_word): Remove write-only 'found_quote'
	local.