Remove reset_symtab_globals

Remove reset_symtab_globals

This patch arranges for the remaining buildsym global --
buildsym_compunit -- to only be cleared by the wrapper functions, not
by methods on struct buildsym_compunit.  In the process,
reset_symtab_globals is removed.

gdb/ChangeLog
2018-07-20  Tom Tromey  <tom@tromey.com>

	* buildsym.c (reset_symtab_globals): Remove.
	(buildsym_compunit::end_symtab_from_static_block): Update.
	(buildsym_compunit::augment_type_symtab): Update.
	(end_symtab_from_static_block): Call free_buildsym_compunit.
	(augment_type_symtab, end_symtab, end_expandable_symtab):
	Likewise.