proc: remove useless d_is_dir() check

Remove the d_is_dir() check from tgid_pidfd_to_pid().

It is pointless since you should never get &proc_tgid_base_operations
for f_op on a non-directory.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Christian Brauner <christian@brauner.io>