[JENKINS-23538] Remove support for Legacy Sorter

 * Removed the old Legacy Sorter
 * Removed various legacy-checks
 * Deprecated PrioritySorterJobProperty, kept enable conversion
 * Removed html-content for PrioritySorterJobProperty
 * Updated configuration page
Updated comments referring to 2.7

 * replaced with 2.x as 3.0 is release target