app: remove unclear fix from gimp_tool_options_editor_tool_changed()

It was introduced in e7ade3f8eca30edb63db4dde075540a3676aea67. The
g_return_if_fail() is IMO useless, the actual fix (which I kept) was
pulling the call to gimp_tool_options_editor_presets_update() inside
the if().
app: don't ref the tool options GUI in gimp_tool_options_editor_dispose()

They are kept alive by a ref owned by GimpToolOptions, refing them
here made them all being leaked on exit.