build: remove now unneeded platform-server package extension

The `@angular/platform-server` package has been updated to correctly
include a peer dependency on `rxjs` and no longer requires a custom
package extension defined.

Author: clydin

package.json

@@ -167,11 +167,6 @@
         "peerDependencies": {
           "protobufjs": "*"
         }
-      },
-      "@angular/platform-server": {
-        "peerDependencies": {
-          "rxjs": "*"
-        }
       }
     }
   },