BUG: TreeDropdownField remove call to get value on search

This call was placing the id of the currently selected record into the
search box. Related to
https://github.com/silverstripe/silverstripe-framework/commit/93ea066f53
d5d2b2a19cf0dd2e9479a3fc5796f7
Merge pull request #2401 from adrexia/tree-value

BUG: TreeDropdownField remove call to get value on search