headers: remove redundant GL token from GL wrapper

Removing GL_FRAMEBUFFER_FLIP_Y_MESA token from glheader.h as it is now
provided by glext.h

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>
22 jobs for !1482 with amend-framebuffer-flip-y-changes in 18 minutes and 53 seconds (queued for 27 seconds)
detached
Status Name Job ID Coverage
  Containers Build
passed debian #570908

00:00:22

 
  Build+Test
passed meson-arm64 #570917

00:02:33

passed meson-armhf #570916

00:02:49

passed meson-clang #570911

00:09:31

passed meson-clover #570914

00:10:16

passed meson-i386 #570918

00:03:37

passed meson-main #570909

00:11:06

passed meson-swr-glvnd #570910

00:06:52

passed meson-vulkan #570915

00:03:10

passed scons-llvm #570920

00:04:20

passed scons-nollvm #570919

00:04:23

passed scons-swr #570912

00:04:28

passed scons-win64 #570913

00:07:18

 
  Test
passed test-llvmpipe-gles2 1/4 #570921

00:03:39

passed test-llvmpipe-gles2 2/4 #570922

00:03:50

passed test-llvmpipe-gles2 3/4 #570923

00:03:42

passed test-llvmpipe-gles2 4/4 #570924

00:03:44

passed test-softpipe-gles2 1/4 #570925

00:01:28

passed test-softpipe-gles2 2/4 #570926

00:01:38

passed test-softpipe-gles2 3/4 #570927

00:01:30

passed test-softpipe-gles2 4/4 #570928

00:01:33

passed test-softpipe-gles3-limited #570929

00:03:54