SSE2 and SSE4.2 builds use AVX2 instructions

[AndrewBetson]

While trying to help an acquaintance get VJolt working on their TF2 server (which is hosted on a machine using an Intel Xeon E5-2690 v2, which doesn't support AVX2) I noticed that the binaries output by the SSE2 and SSE4.2 projects use AVX2 instructions, as shown by elfx86exts in the attachment below.

I believe this is caused by the joltphysics_avx2.vpc script setting the VOLT_AVX2 conditional and not unsetting it, causing the SSE2 and SSE4.2 projects to also use the AVX2-specific settings defined in jolyphysics_settings.vpc