Revert "rtnetlink: Allow deleting FDB entries in user namespace"

This reverts commit 509da346 which is
commit bf29555f upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Id85d10ca56c12b015364a2440c540f6d89203366
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>