Fix long double random

Reference issue

https://oss-fuzz-build-logs.storage.googleapis.com/log-7399495d-a3c8-4ae9-887f-45268bc96304.txt

What does this implement/fix?

When we fall back to double for unsupported long double configurations (e.g. double-double), we need to use std::numeric_limits<double>::digits-1 for the mantissa bits.

Also got rid of redundant static asserts that will never be triggered as pointed out by @ChipKerchner

Additional information