MB-9132: Remove "cover" make target

Newer version of Etap don't support creating cover
reports.

I'm not aware that we actually use this make target.

Change-Id: Icb3b363fa8c37819439bf6c3ad4086f204e82c7d
Reviewed-on: http://review.couchbase.org/29024
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>