Remove references to response file argument in CommandLine.rst

Summary:
The documentation refers to a boolean that controls whether response files are
handled, but this is incorrect. Since r165535, response files are always
enabled.

Reviewers: compnerd, rafael

Reviewed By: compnerd

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38095

llvm-svn: 313830