Introduce htab_delete_entry

Introduce htab_delete_entry

In a bigger series I'm working on, it is convenient to have a
libiberty hash table that manages objects allocated with 'new'.  To
make this simpler, I wrote a small template function to serve as a
concise wrapper.  Then I realized that this could be reused in a few
other places.

gdb/ChangeLog
2021-05-26  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (allocate_type_unit_groups_table)
	(handle_DW_AT_stmt_list, allocate_dwo_file_hash_table): Use
	htab_delete_entry.
	(free_line_header_voidp): Remove.
	* completer.c
	(completion_tracker::completion_hash_entry::deleter): Remove.
	(completion_tracker::discard_completions): Use htab_delete_entry.
	* utils.h (htab_delete_entry): New template function.