plugin-base: remove `MM_PLUGIN_SUPPORTS_PORT_IN_PROGRESS' status

Support checks are fully asynchronous and result is always reported when the
check is considered ready, so in-progress replies to
`mm_plugin_supports_port_finish()' don't make any sense.

src/mm-plugin.h

@@ -49,7 +49,6 @@ typedef void (*MMSupportsPortResultFunc) (MMPlugin *plugin,
 
 typedef enum {
     MM_PLUGIN_SUPPORTS_PORT_UNSUPPORTED = 0x0,
-    MM_PLUGIN_SUPPORTS_PORT_IN_PROGRESS,
     MM_PLUGIN_SUPPORTS_PORT_DEFER,
     MM_PLUGIN_SUPPORTS_PORT_SUPPORTED
 } MMPluginSupportsResult;