input/{file,archive}: remove the dummy InputPlugin instances

These have no use, exception for creating an artificial dependency
from input_plugins[] to all archive plugins.

src/input/plugins/FileInputPlugin.hxx

@@ -26,8 +26,6 @@ class Path;
 class Mutex;
 class Cond;
 
-extern const struct InputPlugin input_plugin_file;
-
 InputStreamPtr
 OpenFileInputStream(Path path,
 		    Mutex &mutex, Cond &cond);