[Driver][Mips] Remove flags which is not used in multi-library selection.

No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213142 91177308-0d34-0410-b5e6-96231b3b80d8
[Driver][Mips] Reduce code duplication - use existing function
getMipsCPUAndABI() to get MIPS ABI name during multi-library selection.

No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213143 91177308-0d34-0410-b5e6-96231b3b80d8