Remove from base obsolete CPP for integer-gmp
- GHC.Natural now exports the same functions (regardless of integer backend)
- remove unnecessary CPP around instances
- remove the 'OPTIMISE_INTEGER_GCD_LCM' flag - almost all of those optimizations now work regardless of which integer backend is used
Note that some CPP still remains for situations where there are backend-specific
optimization hacks (like a more efficient GMP-only gcd for Int# and Word#).