target-arm/translate-a64.c: Correct unallocated checks for ldst_excl

The checks for the unallocated encodings in the ldst_excl group
(exclusives and load-acquire/store-release) were not correct. This
error meant that in turn we ended up with code attempting to handle
the non-existent case of "non-exclusive load-acquire/store-release
pair". Delete that broken and now unreachable code.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Sergey Fedorov <serge.fdrv@gmail.com>
