common: Remove unused functions

common: Remove unused functions

Functions like _mount_opts(), _mkfs_opts() and _fsck_opts() are
defined both in common/rc and common/config while used only in
common/config.

Remove those functions from common/rc and update _mount_opts() to match
the superior version of the function.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>