refs: remove `PACK_REFS_ALL` flag

refs: remove `PACK_REFS_ALL` flag

The intent of the `PACK_REFS_ALL` flag is to ask the backend to compact
all refs instead of only a subset of them. Thus, this flag gets passed
down to `refs_pack_refs()` via `struct pack_refs_opts::flags`.

But starting with 4fe42f326e (pack-refs: teach pack-refs --include
option, 2023-05-12), the flag's semantics have changed. Instead of being
handled by the respective backends, this flag is now getting handled by
the callers of `refs_pack_refs()` which will add a single glob ("*") to
the list of refs-to-be-packed. Thus, the flag serves no purpose to the
ref backends anymore.

Remove the flag and replace it with a local variable.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>