Remove elements from complaint_series

Remove elements from complaint_series

I couldn't find a way to get complaints to use a couple of cases, and
the difference between the actual printed output for these cases was
minimal anyway.  So, this patch removes a couple of constants from
complaint_series, plus the associated code.

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

	* complaints.c (enum complaint_series): Remove FIRST_MESSAGE,
	SUBSEQUENT_MESSAGE.
	(vcomplaint, clear_complaints): Update.
	(symfile_explanations): Remove some messages.

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

	* gdb.gdb/complaints.exp (test_serial_complaints): Remove.
	(test_short_complaints): Update.