Issue #10708: Don't load gegl:nop filters

This patch attempts to resolve #10708 (closed) based on @Jehan's comment in #10708 (comment 2188730)

In earlier development, we accidentally allowed users to save gegl:nop filters. Since they are valid operations, they were not triggering our unsupported operations delete code.

This patch adds a second check to see if the operation is a nop, and deletes it as well.

This patch also adds a check to make sure a filter is in the filter stack before reordering it. This was revealed by the nop bug, so it makes sense to patch both at once.