engines/io_uring: remove unnecessary SQ full check

engines/io_uring: remove unnecessary SQ full check

fio_ioring_queue() bails out if the SQ's tail + 1 == head. This will
always be false, since 0 <= tail - head <= entries. Probably it was
meant to check whether the SQ is full, i.e. tail == head + entries.
(The head index should be loaded with acquire ordering in that case.)
Checking for a full SQ isn't necessary anyways, as the prior check for
ld->queued == td->o.iodepth already ensures the SQ isn't full.

So remove the unnecessary and misleading tail + 1 == head check.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>