Remove streq_hash in favor of htab_eq_string

Remove streq_hash in favor of htab_eq_string

Now that libiberty includes htab_eq_string, we can remove the
identical function from gdb.

gdb/ChangeLog
2021-05-07  Tom Tromey  <tom@tromey.com>

	* breakpoint.c (ambiguous_names_p): Use htab_eq_string.
	* utils.c (streq_hash): Remove.
	* utils.h (streq_hash): Don't declare.
	* completer.c (completion_tracker::discard_completions): Update
	comment.
	* ada-lang.c (_initialize_ada_language): Use htab_eq_string.