[CodeGen] Remove redundant declaration MIRCanonicalizerID (NFC)

Note that MIRCanonicalizerID is declared in
llvm/include/llvm/CodeGen/Passes.h, which MIRCanonicalizerPass.cpp
includes.

Identified with readability-redundant-declaration.

llvm/lib/CodeGen/MIRCanonicalizerPass.cpp

@@ -38,10 +38,6 @@
 
 using namespace llvm;
 
-namespace llvm {
-extern char &MIRCanonicalizerID;
-} // namespace llvm
-
 #define DEBUG_TYPE "mir-canonicalizer"
 
 static cl::opt<unsigned>