clang-modernize: Remove -headers option

clang-modernize can now transform headers properly and the experimental
-headers option is no longer necessary.

Remember, at least -include is necessary for indicating which headers
are allowed to be changed.

Differential Revision: http://llvm-reviews.chandlerc.com/D1610



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@190158 91177308-0d34-0410-b5e6-96231b3b80d8