Trackable URLs: remove destination URL size

Hi,

In CRM_Mailing_BAO_TrackableURL, the destination URL size limit is hardcoded and can’t be over 254 chars (more information in CRM-14730). Now that `civicrm_mailing_trackable_url`’s `url` field is now `text` (CRM-15740), this hardcoded limit does not makes sense anymore.

Associated PR is coming.

Olivier;