Second attempt to fix Sphinx bot

The fix in r282148 was not enough to fix the following error:
/home/llvmbb/llvm-build-dir/clang-sphinx-docs/llvm/src/tools/clang/docs/CommandGuide/clang.rst:338:
WARNING: unknown option: -flto=full

on the sphinx bot:
  http://lab.llvm.org:8011/builders/clang-sphinx-docs/builds/16313

(not reproducible locally).

This time, simply remove the option reference.

llvm-svn: 282151