linux: Remove assembly umount2 implementation

Only alpha and ia64 do not support __NR_umount2 (defined as
__NR_umount), but recent kernel fixes (74cd2184833f for ia64, and
12b57c5c70f39 for alpha) add the required alias.

Checked with a build against all affected ABIs.

Reviewed-by: Florian Weimer <fweimer@redhat.com>