Name hash fixups: export (and rename) remove_hash_entry

Name hash fixups: export (and rename) remove_hash_entry

This makes the name hash removal function (which really just sets the
bit that disables lookups of it) available to external routines, and
makes read_cache_unmerged() use it when it drops an unmerged entry from
the index.

It's renamed to remove_index_entry(), and we drop the (unused) 'istate'
argument.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>