ext_external_objects: Delete incorrectly git-added generated code.

These are generated by the CMakeLists.gl.txt from the corresponding
.frag or .vert file, so they should not be checked in to git.

Reviewed-by: Eleni Maria Stea <elene.mst@gmail.com>
Part-of: <mesa/piglit!515>
10 jobs for vk-image-display-fix in 8 minutes and 31 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Container
passed debian #13782179

00:00:28

passed windows #13782180
windows shell 1809

00:00:11

 
  Build
passed build-debian #13782181

00:03:51

passed build-debian-mingw: [mingw32] #13782182

00:03:56

passed build-debian-mingw: [mingw64] #13782183

00:05:06

passed build-windows #13782184
windows docker 1809

00:08:01

passed py.test #13782185

00:00:27

passed tox: [3, 6] #13782186

00:05:31

passed tox: [3, 7] #13782187

00:02:32

passed tox: [3, 8] #13782188

00:02:27