Remove struct complaints

Remove struct complaints

struct complaints now just holds a single enum value, so remove it and
symfile_complaint_book and replace it with a global variable.

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

	* complaints.c (struct complaints): Remove.
	(symfile_complaint_book): Remove.
	(series): New global.
	(complaint_internal): Update.
	(clear_complaints): Update.