remove empty string (fixes a warning)

Ulfex warning: inavlid po attributes extracted from /home/timar/libreoffice-master/sysui/desktop/share/launcher_unityquicklist.ulf
No string specified!
GroupID: startcenter

Change-Id: I39b08feb8bb691da17a7a7ed5d17411720ae1d6a
Reviewed-on: https://gerrit.libreoffice.org/3909Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Tested-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>

sysui/desktop/share/launcher_unityquicklist.ulf

@@ -15,6 +15,3 @@ en-US = "New Formula"
 
 [draw]
 en-US = "New Drawing"
-
-[startcenter]
-en-US = ""