proc: Remove the now unnecessary internal mount of proc

There remains no more code in the kernel using pids_ns->proc_mnt,
therefore remove it from the kernel.

The big benefit of this change is that one of the most error prone and
tricky parts of the pid namespace implementation, maintaining kernel
mounts of proc is removed.

In addition removing the unnecessary complexity of the kernel mount
fixes a regression that caused the proc mount options to be ignored.
Now that the initial mount of proc comes from userspace, those mount
options are again honored.  This fixes Android's usage of the proc
hidepid option.

Reported-by: Alistair Strachan <astrachan@google.com>
Fixes: e94591d0 ("proc: Convert proc_mount to use mount_ns.")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>