reftable/stack: drop only use of `get_locked_file_path()`

reftable/stack: drop only use of `get_locked_file_path()`

We've got a single callsite where we call `get_locked_file_path()`. As
we're about to convert our usage of the lockfile subsystem to instead be
used via a compatibility shim we'd have to implement more logic for this
single callsite. While that would be okay if Git was the only supposed
user of the reftable library, it's a bit more awkward when considering
that we have to reimplement this functionality for every user of the
library eventually.

Refactor the code such that we don't call `get_locked_file_path()`
anymore.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>