[AMDGPU] Remove "using namespace" from a header. NFC. (#154776)

Author: jayfoad

llvm/lib/Target/AMDGPU/AMDGPUMCInstLower.h

@@ -24,8 +24,6 @@ class AsmPrinter;
 class MCContext;
 } // namespace llvm
 
-using namespace llvm;
-
 class AMDGPUMCInstLower {
   MCContext &Ctx;
   const TargetSubtargetInfo &ST;