[Vectorize] Remove a redundant declaration (NFC) (#167188)

EnableVPlanNativePath is declared in LoopVectorizationPlanner.h.

Identified with readability-redundant-declaration.

Author: kazutakahirata

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -52,10 +52,6 @@
 using namespace llvm;
 using namespace llvm::VPlanPatternMatch;
 
-namespace llvm {
-extern cl::opt<bool> EnableVPlanNativePath;
-}
-
 /// @{
 /// Metadata attribute names
 const char LLVMLoopVectorizeFollowupAll[] = "llvm.loop.vectorize.followup_all";