mesa: remove unused gl_texture_image::DriverData field

Was only used by some older/removed DRI drivers.

src/mesa/main/mipmap.c

@@ -1816,7 +1816,6 @@ generate_mipmap_uncompressed(struct gl_context *ctx, GLenum target,
       _mesa_init_teximage_fields(ctx, target, dstImage, dstWidth, dstHeight,
                                  dstDepth, border, srcImage->InternalFormat,
                                  srcImage->TexFormat);
-      dstImage->DriverData = NULL;
 
       /* Alloc storage for new texture image */
       if (!ctx->Driver.AllocTextureImageBuffer(ctx, dstImage,

src/mesa/main/mtypes.h

@@ -1269,13 +1269,6 @@ struct gl_texture_image
    GLuint *ImageOffsets;        /**< if 3D texture: array [Depth] of offsets to
                                      each 2D slice in 'Data', in texels */
    GLvoid *Data;		/**< Image data, accessed via FetchTexel() */
-
-   /**
-    * \name For device driver:
-    */
-   /*@{*/
-   void *DriverData;		/**< Arbitrary device driver data */
-   /*@}*/
 };