Remove unnecessary imports from PostService

Will make possible to move `PostService` to WordPressData without
carrying `CommentService`.

Author: mokagio

WordPress/Classes/Services/PostService.m

@@ -1,8 +1,5 @@
 #import "PostService.h"
 #import "PostCategory.h"
-#import "PostCategoryService.h"
-#import "CommentService.h"
-#import "MediaService.h"
 #import "Media.h"
 #ifdef KEYSTONE
 #import "Keystone-Swift.h"