Revert "[OpenCL] Add supported OpenCL extensions to target info."

Revert r269431 due to build failure caused by warning msg:

  llvm/tools/clang/lib/Basic/Targets.cpp:2090:9: error: 'setSupportedOpenCLOpts' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
   void setSupportedOpenCLOpts() {

llvm-svn: 269435