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

[brad]

Summary

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

clang/lib/Driver/ToolChains/CSKYToolChain.h

 namespace driver {
 namespace toolchains {
 
 class LLVM_LIBRARY_VISIBILITY CSKYToolChain : public Generic_ELF {
 public:
   CSKYToolChain(const Driver &D, const llvm::Triple &Triple,
                 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;
   RuntimeLibType GetDefaultRuntimeLibType() const override;
   UnwindLibType GetUnwindLibType(const llvm::opt::ArgList &Args) const override;
   void
   AddClangSystemIncludeArgs(const llvm::opt::ArgList &DriverArgs,
                             llvm::opt::ArgStringList &CC1Args) const override;