plugin/builder: remove unused to_json method

Since the switch to requests, with its support for dictionary to
json conversion, this method is no longer necessary.

plugins/builder/osbuild.py

@@ -17,7 +17,6 @@ alone client for composer's API.
 
 import configparser
 import enum
-import json
 import sys
 import time
 import urllib.parse
@@ -117,12 +116,6 @@ class ComposeRequest:
             ]
         }
 
-    def to_json(self, encoding=None):
-        data = json.dumps(self.as_dict())
-        if encoding:
-            data = data.encode('utf-8')
-        return data
-
 
 class ImageStatus(enum.Enum):
     SUCCESS = "success"