CmdExport: Do not use context

Per the design of the context feature, 'task export' is not
supposed to leverage context. This is a regression from 2.4.4.

src/commands/CmdExport.cpp

@@ -42,7 +42,7 @@ CmdExport::CmdExport ()
   _read_only             = true;
   _displays_id           = true;
   _needs_gc              = true;
-  _uses_context          = true;
+  _uses_context          = false;
   _accepts_filter        = true;
   _accepts_modifications = false;
   _accepts_miscellaneous = false;