refactor(@angular-devkit/build-angular): extract-18n buildTarget option is no longer required

The `extract-i18n` option schema has been updated to reflect that the
`buildTarget` option is not required. The option value will default
to the `build` target of the containing project. This removes the need
for an additional option and character count within the `angular.json`
file for the project. This change also applies to the deprecated
`browserTarget` option.

(cherry picked from commit 4d679a8)

Author: clydin

packages/angular_devkit/build_angular/src/builders/extract-i18n/schema.json

@@ -35,6 +35,5 @@
       "description": "Name of the file to output."
     }
   },
-  "additionalProperties": false,
-  "anyOf": [{ "required": ["buildTarget"] }, { "required": ["browserTarget"] }]
+  "additionalProperties": false
 }