cocci rules: remove unused "F" metavariable from pending rule

cocci rules: remove unused "F" metavariable from pending rule

Fix an issue with a rule added in 9b45f499818 (object-store: prepare
has_{sha1, object}_file to handle any repo, 2018-11-13). We've been
spewing out this warning into our $@.log since that rule was added:

	warning: rule starting on line 21: metavariable F not used in the - or context code

We should do a better job of scouring our coccinelle log files for
such issues, but for now let's fix this as a one-off.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>