Move repository_format_partial_clone to promisor-remote.c

Now that we have has_promisor_remote() and can use many
promisor remotes, let's hide repository_format_partial_clone
as a static in promisor-remote.c to avoid it being use
for anything other than managing backward compatibility.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.c

@@ -31,7 +31,6 @@ int warn_ambiguous_refs = 1;
 int warn_on_object_refname_ambiguity = 1;
 int ref_paranoia = -1;
 int repository_format_precious_objects;
-char *repository_format_partial_clone;
 const char *core_partial_clone_filter_default;
 int repository_format_worktree_config;
 const char *git_commit_encoding;