i965: remove unused variable at intel_miptree_create_for_teximage

After commit "i965: Fix calculation of the image height at start level", it is
not needed. This commit removes the "warning: unused variable ‘i’" warning.

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>

src/mesa/drivers/dri/i965/intel_tex_image.c

@@ -40,7 +40,6 @@ intel_miptree_create_for_teximage(struct brw_context *brw,
 {
    GLuint lastLevel;
    int width, height, depth;
-   GLuint i;
 
    intel_get_image_dims(&intelImage->base.Base, &width, &height, &depth);