Revert "io_uring: gate REQ_F_ISREG on !S_ANON_INODE as well"

This reverts commit 6f11adcc.

The problematic commit was fixed in mainline, so the work-around in
io_uring can be removed at this point. Anonymous inodes no longer
pretend to be regular files after:

1e7ab6f6 ("anon_inode: rework assertions")

Signed-off-by: Jens Axboe <axboe@kernel.dk>