cache_or_unpack_entry: drop keep_cache parameter

cache_or_unpack_entry: drop keep_cache parameter

There is only one caller of cache_or_unpack_entry() and it
always passes 1 for the keep_cache parameter. We can
simplify it by dropping the "!keep_cache" case.

Another call, which did pass 0, was dropped in abe601b
(sha1_file: remove recursion in unpack_entry, 2013-03-27),
as unpack_entry() now does more complicated things than a
simple unpack when there is a cache miss.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>