Remove unused property theme

theme doesn't exist in the documentation of DiffPreferencesInfo
and there are no usages with our codebase as well as of now.
Dark theme is currently being maintained via local storage.

Change-Id: Ideb20c47905f61dcda71500db56a2b846ec2dec6
diff --git a/polygerrit-ui/app/api/diff.ts b/polygerrit-ui/app/api/diff.ts
index 7ee68b7..39a4441 100644
--- a/polygerrit-ui/app/api/diff.ts
+++ b/polygerrit-ui/app/api/diff.ts
@@ -174,8 +174,6 @@
   font_size: number;
   // TODO: Missing documentation
   show_file_comment_button?: boolean;
-  // TODO: Missing documentation
-  theme?: string;
 }
 
 export declare interface RenderPreferences {
diff --git a/polygerrit-ui/app/constants/constants.ts b/polygerrit-ui/app/constants/constants.ts
index 03d5000..d78b3cd 100644
--- a/polygerrit-ui/app/constants/constants.ts
+++ b/polygerrit-ui/app/constants/constants.ts
@@ -416,7 +416,6 @@
     show_whitespace_errors: true,
     syntax_highlighting: true,
     tab_size: 8,
-    theme: 'DEFAULT',
   };
 }
 
diff --git a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface_test.js b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface_test.js
index fbd675f..8c29c2f 100644
--- a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface_test.js
+++ b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface_test.js
@@ -409,7 +409,6 @@
       assert.equal(obj.show_whitespace_errors, true);
       assert.equal(obj.syntax_highlighting, true);
       assert.equal(obj.tab_size, 8);
-      assert.equal(obj.theme, 'DEFAULT');
     });
   });