Remove the "If only executable code is distributed..." paragraph from
the license text. Everything now uses a normal MIT license.
The only code from Khronos that's included in libglvnd is the EGL/GL
header and XML files, which do not contain that paragraph.
Fixes https://gitlab.freedesktop.org/glvnd/libglvnd/-/issues/221
This subtest verifies that the GLX API library's implementation of
glXGetClientString() (which takes the "union" (concatenation) of each
vendor's client string) works as expected.