Revert "Fix how LLVMOPTIONALCOMPONENTS is passed to llvm-build"

This reverts commit r334543.

My understanding is, that commit is intended to make the llvm-build
invocation have a correct "--enable-optional-components" value, but:
- it already has a value: it's quoted in the command line a few lines
  below, and, if I hack llvm-build to print sys.argv, it does look correct:
    -- llvm-build output: ['.../utils/llvm-build/llvm-build',
      '--native-target', 'X86', '--enable-targets', 'X86;ARM;AArch64',
      '--enable-optional-components', '',
      '--write-library-table',
      '.../build/tools/llvm-config/LibraryDependencies.inc',
      '--write-cmake-fragment', '.../build/LLVMBuild.cmake']
- the " " string seems to evaluate to TRUE in CMake (*sigh*), so this
  basically force-enables LLVM_USE_INTEL_JITEVENTS, regardless of the
  value of the option.
  On Darwin, JITEvents is not supported, so this bypasses that OS check
  but is guaranteed to fail later.

llvm-svn: 334566