[StaticAnalyzer] Remove unused declaration handleLVectorSplat

The corresponding function definition was removed by:

  commit 55662b2
  Author: Balazs Benics <[email protected]>
  Date:   Thu Jul 1 10:54:28 2021 +0200

Author: kazutakahirata

clang/include/clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h

@@ -607,12 +607,6 @@ class ExprEngine {
                                       StmtNodeBuilder &Bldr,
                                       ExplodedNode *Pred);
 
-  ProgramStateRef handleLVectorSplat(ProgramStateRef state,
-                                     const LocationContext *LCtx,
-                                     const CastExpr *CastE,
-                                     StmtNodeBuilder &Bldr,
-                                     ExplodedNode *Pred);
-
   void handleUOExtension(ExplodedNodeSet::iterator I,
                          const UnaryOperator* U,
                          StmtNodeBuilder &Bldr);