Fix the FLASHINFER_ENABLE_FP8 flag in pyproject.toml and jit.cpp_ext

[diptorupd]

Look at Options.cmake setting FLASHINFER_ENABLE_FP8 sets the FLASHINFER_ENABLE_FP8_E4M3 and FLASHINFER_ENABLE_FP8_E5M2 to true. So, we should only use the FLASHINFER_ENABLE_FP8 flag.

Originally posted by @diptorupd in #40