Remove unused fields from ReleaseMixin

These are handled through repos and services instead

release_mixin.py

@@ -31,16 +31,9 @@ class ReleaseMixin(DevopsBuild):
         self.release_type_repo = ReleaseTypeRepository(self.git_api)
         self.version_repo = VersionRepository(self.config_file)
         self.release_repo = ReleaseRepository(self.version_repo, self.release_type_repo, self.main_branch)
-        self.release_version = None
-        self.bump_version = None
-        self.commit_string = None
 
     def prepare_release(self):
         prepare_release_service = PrepareReleaseService(self.release_repo)
-
-        if self.release_version is None or self.bump_version is None:
-            raise Exception('prepare_release was called before init_release')
-        
         # prepare_release_service.run_tests() # not implemented
         prepare_release_service.write_and_commit_release()
         prepare_release_service.write_and_commit_bump()