Remove struct complain

Remove struct complain

At this point, struct complain is just holds a key, a value, and a
"next" pointer to form a linked list.  It's simpler to replace this
with an unordered map.

gdb/ChangeLog
2018-05-23  Tom Tromey  <tom@tromey.com>

	* complaints.c (counters): New global.
	(struct complain): Remove.
	(struct complaints) <root>: Remove.
	(complaint_sentinel): Remove.
	(symfile_complaint_book): Update.
	(find_complaint) Remove.
	(complaint_internal, clear_complaints): Update.

gdb/testsuite/ChangeLog
2018-05-23  Tom Tromey  <tom@tromey.com>

	* gdb.gdb/complaints.exp (test_initial_complaints): Simplify.