[RISCV] Remove redundant enabling of IAS for Clang, NFC

Generic_GCC::IsIntegratedAssemblerDefault() already takes care of RISCV.

Reviewed By: kito-cheng, MaskRay

Differential Revision: https://reviews.llvm.org/D123097

clang/lib/Driver/ToolChains/RISCVToolchain.h

@@ -22,7 +22,6 @@ public:
                  const llvm::opt::ArgList &Args);
 
   static bool hasGCCToolchain(const Driver &D, const llvm::opt::ArgList &Args);
-  bool IsIntegratedAssemblerDefault() const override { return true; }
   void addClangTargetOptions(const llvm::opt::ArgList &DriverArgs,
                              llvm::opt::ArgStringList &CC1Args,
                              Action::OffloadKind) const override;