InitRootFS was added in #1477 for potential future use but has no
callers either inside or outside this repository. Remove it along
with the associated Mounter interface, initializerFunc type,
createInitLayer helper, and the platform-specific init files that
only existed to support it.
Fixes#13238
Signed-off-by: Sanil2108 <sanilkhurana7@gmail.com>