tools: engine-config.properties: remove left over line.

commit 116fef64 added meaningful description of the WebSocketProxy
different options. However it seems as though the old description
(value) was listed as a new unknown key.

This causes the following line to be outputed as a value:
null:  (Value Type: String)

Change-Id: I4a991e5aeabdf2195624a2899e6f2bf62e49296a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1116392
Signed-off-by: Mooli Tayer <mtayer@redhat.com>