Remove dub.selections.json

For applications, it makes sense to have dub.selections.json to lock in what version to use.
However, for a library like std_data_json, we can just use the version in dub.json.

Author: Geod24

.gitignore

@@ -3,3 +3,6 @@ docs.json
 __dummy.html
 *.[ao]
 *.obj
+
+# Since this is a library, ignore `dub.selections.json`
+dub.selections.json