mojo: Remove MOJO_IMPLEMENTATION define.

This seems to be unused right now.

This can be added later when a mojo_export.h is added, and the define can be
(should?) defined locally to the library instead of being global.

BUG=None
TEST=None
R=darin@chromium.org

Review URL: https://codereview.chromium.org/56763002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232618 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232618