Remove struct explanation

Remove struct explanation

Now that there's only a single reason for a complaint to be emitted,
this removes "struct explanation" and changes vcomplaint to emit the
desired messages directly.

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

	* complaints.c (struct complaints) <explanation>: Remove.
	(symfile_explanations): Remove.
	(symfile_complaint_book): Update.
	(vcomplaint): Update.
	(struct explanation): Remove.