environment: stop storing "core.preferSymlinkRefs" globally

Same as the preceding commit, storing the "core.preferSymlinkRefs" value
globally is misdesigned as this setting may be set per repository.

There is only a single user of this value anyway, namely the "files"
backend. So let's just remove the global variable and read the value of
this setting when initializing the backend.

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

environment.c

@@ -34,7 +34,6 @@ int has_symlinks = 1;
 int minimum_abbrev = 4, default_abbrev = -1;
 int ignore_case;
 int assume_unchanged;
-int prefer_symlink_refs;
 int is_bare_repository_cfg = -1; /* unspecified */
 int warn_ambiguous_refs = 1;
 int warn_on_object_refname_ambiguity = 1;