Use htab_up in completion_tracker

Use htab_up in completion_tracker

This changes completion_tracker to use htab_up, rather than explicit
calls to htab_delete.

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

	* completer.c (completion_tracker::discard_completions)
	(completion_tracker::~completion_tracker)
	(completion_tracker::maybe_add_completion)
	(completion_tracker::remove_completion)
	(completion_tracker::recompute_lowest_common_denominator)
	(completion_tracker::build_completion_result): Update.
	* completer.h (class completion_tracker) <have_completions>:
	Update.
	<m_entries_hash>: Now htab_up.