[Sema] Fix a warning

This patch fixes:

  clang/lib/Sema/AnalysisBasedWarnings.cpp:2269:21: error: unused
  variable 'subExpr' [-Werror,-Wunused-variable]

(cherry picked from commit 73ff017)

Author: kazutakahirata

clang/lib/Sema/AnalysisBasedWarnings.cpp

@@ -2242,8 +2242,6 @@ class UnsafeBufferUsageReporter : public UnsafeBufferUsageHandler {
         if (!isa<PointerType>(destType))
           return;
 
-        const Expr *subExpr = ECE->getSubExpr();
-
         const uint64_t dSize =
             Ctx.getTypeSize(destType.getTypePtr()->getPointeeType());