Multi-statement detection works differently on non-AVX2 CPUs

While investigating MXS-4817, I found out that the handling of trailing END tokens is different between the AVX2 and the generic versions of the multi-statement detection code. The AVX2 version does not detect the trailing ends whereas the generic one does.

The behavior of the generic one is wrong as it also discards true multi-statement commands executed with BEGIN NOT ATOMIC. Thus, both this bug and MXS-4817 will be fixed by simply removing the detection from the generic version.