Use htab_up in linespec.c

Use htab_up in linespec.c

This changes linespec.c to use htab_up rather than explicit calls to
htab_delete.  Note that a use still exists in this file, because
linespec_state hasn't been converted to have a real destructor.

gdb/ChangeLog
2020-09-17  Tom Tromey  <tom@tromey.com>

	* linespec.c (class decode_compound_collector)
	<~decode_compound_collector>: Remove.
	<m_unique_syms>: Now htab_up.
	(decode_compound_collector::operator ()): Update.
	(class symtab_collector) <~symtab_collector>: Remove.
	<m_symtab_table>: Now htab_up.
	(symtab_collector::operator ()): Update.