[NFC] Remove unused-but-set global variables (#189315)
Remove four global variables that are set but never read to fix
-Wunused-but-set-global warnings:
- `MFMAChainLength` in AMDGPUIGroupLP.cpp
- `Wide` in llvm-objdump.cpp
- `SaveTemps` in ClangSYCLLinker.cpp
- `DeprecatedDriverCommand` in ClangScanDeps.cpp
Follow up to #178342
