Remove Worldcat/Backend::similar()

* VuFindSearch/Backend/WorldCat/Backend.php (similar):
  Removed. Not longer required by BackendInterface.
Move similar() to MoreLikeThis feature interface

* VuFindSearch/Backend/BackendInterface.php (similar):
  Removed. Defined by MoreLikeThis feature interface.
* VuFindSearch/Backend/Solr/Backend.php: Implement feature
  interface.
* VuFindSearch/Feature/MoreLikeThis.php: New file. MoreLikeThis
  feature.