[LegalizeTypes] Remove untested code from ExpandIntOp_UINT_TO_FP

This code is untested in tree because the "APFloat::semanticsPrecision(sem) >= SrcVT.getSizeInBits() - 1" check is false for most combinations for int and fp types except maybe i32 and f64. For that you would need i32 to be an illegal type, but f64 to be legal and have custom handling for legalizing the split sint_to_fp. The precision check itself was added in 2010 to fix a double rounding issue in the algorithm that would occur if the sint_to_fp was not able to do the conversion without rounding.

Differential Revision: https://reviews.llvm.org/D72728