remove stray declaration of vips_mapimage()
remove stray declaration of vips_mapimage()

We declared vips_mapimage() in histogram.h. This became
vips_case() later in development, but this old decl was left behind in
error.

Thanks @remicollet

See https://github.com/libvips/libvips/issues/1513

jcupitt/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips