[LV] Remove stale completeLoopSkeleton (NFCI).

The function has been removed a while ago, also remove the stable
declaration.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -585,11 +585,6 @@ class InnerLoopVectorizer {
       const SCEV2ValueTy &ExpandedSCEVs,
       std::pair<BasicBlock *, Value *> AdditionalBypass = {nullptr, nullptr});
 
-  /// Complete the loop skeleton by adding debug MDs, creating appropriate
-  /// conditional branches in the middle block, preparing the builder and
-  /// running the verifier. Return the preheader of the completed vector loop.
-  BasicBlock *completeLoopSkeleton();
-
   /// Allow subclasses to override and print debug traces before/after vplan
   /// execution, when trace information is requested.
   virtual void printDebugTracesAtStart(){};