[PowerPC][NFC] Remove unused (and unsupported) fusion feature bits.

[PowerPC][NFC] Remove unused (and unsupported) fusion feature bits.

FeatureFusion bits was first introduced in
https://reviews.llvm.org/rL253724. for add/load integer fusion for P8.
The only use of `hasFusion` was https://reviews.llvm.org/rL255319.

However, this was removed later in https://reviews.llvm.org/rL280440.

So, there is NO any reference to fusion in code now.

Leaving it there is misleading and confusing, so remove it for now.
We can alwasy add back if we ever support fusion in the future.

llvm-svn: 364581