src/app/core: remove unnecessary comment

Remove commented out this.setCitationDOITag() since it is not used
and we use camel case with this.setCitationDoiTag() now anyway.

src/app/core/metadata/head-tag.service.ts

@@ -199,7 +199,6 @@ export class HeadTagService {
     // this.setCitationIssueTag();
     // this.setCitationFirstPageTag();
     // this.setCitationLastPageTag();
-    // this.setCitationDOITag();
     // this.setCitationPMIDTag();
 
     // this.setCitationFullTextTag();