refs/ref-cache.c: remove "mkdir" parameter from find_containing_dir()

refs/ref-cache.c: remove "mkdir" parameter from find_containing_dir()

Remove the "mkdir" parameter from the find_containing_dir() function,
the add_ref_entry() function removed in the preceding commit was its
last user.

Since "mkdir" is always "0" we can also remove the parameter from
search_for_subdir(), which in turn means that we can delete most of
that function.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>