Remove minsym termination

Remove minsym termination

I was curious what used the terminating "null" minimal symbol; and
after looking I could not find anything.  This patch removes
terminate_minimal_symbol_table and the extra minimal symbol that is
allocated for it.

gdb/ChangeLog
2019-03-15  Tom Tromey  <tom@tromey.com>

	* symfile.c (reread_symbols): Update.
	* objfiles.c (objfile::objfile): Update.
	* minsyms.h (terminate_minimal_symbol_table): Don't declare.
	* minsyms.c (lookup_minimal_symbol_by_pc_section): Update
	comment.
	(minimal_symbol_reader::install): Update.
	(terminate_minimal_symbol_table): Remove.
	* jit.c (jit_object_close_impl): Update.