update-index: remove stale fallback code for "--unresolve"

update-index: remove stale fallback code for "--unresolve"

The "update-index --unresolve" is a relatively old feature that was
introduced in Git v1.4.1 (June 2006), which predates the
resolve-undo extension introduced in Git v1.7.0 (February 2010).
The original code that was limited only to work during a merge (and
not during a rebase or a cherry-pick) has been kept as the fallback
codepath to be used as a transition measure.

By now, for more than 10 years we have stored resolve-undo extension
in the index file, and the fallback code way outlived its usefulness.

Remove it, together with two file-scope static global variables.
One of these variables is still used by surviving function, but it
does not have to be a global at all, so move it to local to that
function.

Signed-off-by: Junio C Hamano <gitster@pobox.com>