promisor-remote: remove unused variable

promisor-remote: remove unused variable

The variable core_partial_clone_filter_default has been unused since
fa3d1b63e8 ("promisor-remote: parse remote.*.partialclonefilter",
2019-06-25), when Git was changed to refer to
remote.*.partialclonefilter as the default filter when fetching in a
partial clone, but (perhaps inadvertently) there was no fallback to
core.partialclonefilter.

One alternative is to add the fallback, but the aforementioned change
was made more than a year ago and I have not heard of any complaints
regarding this matter. In addition, there is currently no mention of
core.partialclonefilter in the user documentation. So it seems best to
reaffirm that Git will only support remote.*.partialclonefilter.

Signed-off-by: Jonathan Tan <redacted>
Signed-off-by: Junio C Hamano <redacted>