[PowerPC] Remove the repeated definition for some InstAlias for mtspr/mfspr

Summary:
Below InstAlias have been redefined, this patch is to remove the repeated
definition.
mtdec/mfdec mtsdr1/mfsdr1 mtsrr0/mfsrr0 mtsrr1/mfsrr1 mtasr

Reviewed By: nemanjai, steven.zhang

Differential Revision: https://reviews.llvm.org/D75821