Remove gimp_drawable_replace_buffer(), which is no longer used
anywhere since commits ddb69b77 and
3451ffb6.  This eliminates
redundancy, since all compositing is now done through the layer-
mode code.

Furthermore, gimp_drawable_replace_buffer() used the drawable's
active-component array, whose layout depends on the image mode, as
an argument to gimp_gegl_replace(), which always expects an RGBA
component array, resulting in broken component masking in non-RGB
images.