gdb: use the location_completer for the list command

gdb: use the location_completer for the list command

The "location_completer" function has been available for a long time,
but it was seemingly never used as the completer for the list function.
A quick check through git history shows that a similar completer was
available for the "edit" command but wasn't added to "list" with no
reasoning for it.

I think "list" should use the location_completer, as it is more aware of
the locspec grammar.

Approved-By: Tom Tromey <tom@tromey.com>