mesa: Remove all mention of MESA_FORMAT_CI8

Nothing in Mesa supports color-index textures, and most of the other
infrastructure that could allow such support has already been removed.
This puts the final nail in the coffin.

Also clean out some GL_COLOR_INDEX comments in formats.c.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/main/fbobject.c

@@ -2321,7 +2321,7 @@ _mesa_GetFramebufferAttachmentParameterivEXT(GLenum target, GLenum attachment,
       }
       else {
          gl_format format = att->Renderbuffer->Format;
-         if (format == MESA_FORMAT_CI8 || format == MESA_FORMAT_S8) {
+         if (format == MESA_FORMAT_S8) {
             /* special cases */
             *params = GL_INDEX;
          }