Remove exp.lower-constants

[michael-schwarz]

After goblint/cil#59, CIL is aware of overflows in constant folding and does not fold constants that lead to signed overflows.

This means we can remove the option exp.lower-constants that was needed for overflow warnings to be sound.

TODO:

• Wait for Make constFold aware of overflows and prevent constant folding on overflows cil#59 to be merged
• See if this causes issues with sv-comp NoOverflow benchmarks