Cleanup unnecessary env variables

GEMM and FlashAttention are ran with a number of env variables, this issue is to minimize them.

As suggested in https://github.com/intel/intel-xpu-backend-for-triton/pull/1877#discussion_r1725361191 and https://github.com/intel/intel-xpu-backend-for-triton/pull/1877#discussion_r1725369700.