[clang] Fix a warning

This patch fixes:

  clang/lib/AST/Interp/ByteCodeExprGen.cpp:978:24: warning: variable
  ‘T’ set but not used [-Wunused-but-set-variable]

T and ReturnType were introduced on August 19, 2022 in commit
8e41e6a4ea.

Their last uses were removed on October 13, 2022 in commit
0e754cfadc.

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -974,8 +974,6 @@ bool ByteCodeExprGen<Emitter>::VisitCallExpr(const CallExpr *E) {
     if (Func->isFullyCompiled() && !Func->isConstexpr())
       return false;
 
-    QualType ReturnType = E->getCallReturnType(Ctx.getASTContext());
-    Optional<PrimType> T = classify(ReturnType);
     // Put arguments on the stack.
     for (const auto *Arg : E->arguments()) {
       if (!this->visit(Arg))