Revert "wic/plugins/images/direct: Allow changes in fstab on rootfs"

Revert "wic/plugins/images/direct: Allow changes in fstab on rootfs"

Commit 20d43a2 allows changes in fstab for the / mount. However, if the
fstab contains / already then this results in two entries for / in the
fstab, which is confusing and results in systemd producing errors on
boot:

  systemd-fstab-generator[11101]: Failed to create unit file
  /run/systemd/generator/-.mount, as it already exists. Duplicate entry
  in /etc/fstab?

Wic should be extended to support merging mount points in fstab, but as
we're about to release revert the patch that introduced this change as
it hasn't been in a release yet, and the issue which prompted the patch
in the first place can be revisited post-release.

[ YOCTO #14865 ]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>