No longer use ':add_path' key for '--remove-path'

lib/copyright-header/parser.rb:184 -- Previously trying to split the
:add_path when it may not be supplied during license removal.

Prior to this change, the argument passed to --remove-path is
essentially ignored, and the command will fail if --remove-path is
suppled but --add-path is not.
Merge pull request #7 from caitp/remove-path

Fixes broken --remove-path option