Remove extra paragraph from license text

This removes the "If only executable code is distributed..." paragraph from the license text.

I haven't figured out for sure why that text was added in the first place -- it doesn't appear in the OpenGL or EGL registry repos, which are the only Khronos files that libglvnd includes. It was apparently in some older revisions of the KTX code, but libglvnd doesn't have anything to do with that.

My best guess is that it was copied and pasted from another project, and then further copied through libglvnd from there.