refactor(@angular/build): 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.

(cherry picked from commit 3f8e09c)

Author: clydin

goldens/public-api/angular/build/index.md

@@ -152,7 +152,7 @@ export function executeExtractI18nBuilder(options: ExtractI18nBuilderOptions, co
 
 // @public
 export interface ExtractI18nBuilderOptions {
-    buildTarget: string;
+    buildTarget?: string;
     format?: Format;
     outFile?: string;
     outputPath?: string;

packages/angular/build/src/builders/extract-i18n/schema.json

@@ -29,6 +29,5 @@
       "description": "Name of the file to output."
     }
   },
-  "additionalProperties": false,
-  "required": ["buildTarget"]
+  "additionalProperties": false
 }