[kwinglutils] Init glVersion on gles

The glVersion was not set at all for gles causing any code doing a
hasGLVersion check to fail which means that the compositor doesn't
start at all, because it has a hasGLVersion(2, 0) check.

The complete ifdef is no longer needed. The used additional glGetStringi
is also available in gles 3.0, thus epoxy can handle it quite well
without a need for ifdef.

Unfortunately the version string can be "strange" on GLES. On desktop
it looks like: "3.0 some driver foo", on GLES it could also be:
"OpenGL ES 3.0 some driver bar". Thus to make the logic work we are
first removing any leading "OpenGL ES " and hope that then the version
is encoded just like on GL.