Remove version checks for ml_dtypes >= 0.5.0
This PR removes runtime checks for ml_dtypes >= 0.5.0 related to float8_e8m0fnu and float4_e2m1fn, since version 0.5.0 is now the minimum required version for JAX.
The ml_dtype requirement in JAX was updated to version 0.5.0+ (on Mar 20, 2025) - commit 4b7ead4
- The checks are no longer necessary and simplify the test logic.
- All supported environments are guaranteed to have ml_dtypes >= 0.5.0.
PiperOrigin-RevId: 783511371