Remove support for GL_EXT_shared_texture_palette

Apparently the paletted texture extension is effectively dead:
https://www.opengl.org/registry/specs/EXT/shared_texture_palette.txt
https://www.opengl.org/registry/specs/EXT/paletted_texture.txt

Changes, revision 1.4, March 24, 2004 (Mark Kilgard)
    Document vendor support for this extension; note that future NVIDIA
    GPU designs will not support this extension.
Remove support for GL_SGIS_multitexture

This extension was made obsolete in favor of GL_ARB_multitexture long ago.