environment: stop storing "core.preferSymlinkRefs" globally

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>