Remove unnecessary #extension directive

[vignedev]

This should fix shader compile errors for Mesa drivers on Linux, however I was able to only test this on Intel GPUs. The extension looks to be also enabled automatically in source/blender/gpu/opengl/gl_shader.cc:817.

Fixes #3.

[williamchange]

Can confirm this also works with building on a mac (M1), was crashing whenever I switch to eevee.

[justjulez420]

can confirm this works on AMD GPUs too

[Lateasusual]

Thanks for fixing that, I saw a few people had issues with it :)