shaped-texture: Don't clear mipmaps during painting

This reverts an attempt at saving texture memory that was introduced
recently in 16fa2100. It was misguided because the same texture may be
needed in the next frame if a window has multiple previews visible on
screen at once (gnome-shell's overview). Keeping the mipmaps around
seems to reduce the peak render times of the overview by roughly 5%-10%.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2598>

src/compositor/meta-shaped-texture.c

@@ -635,9 +635,6 @@ do_paint_content (MetaShapedTexture   *stex,
     {
       min_filter = COGL_PIPELINE_FILTER_NEAREST;
       mag_filter = COGL_PIPELINE_FILTER_NEAREST;
-
-      /* Back to normal desktop viewing. Save some memory */
-      meta_texture_mipmap_clear (stex->texture_mipmap);
     }
   else
     {