Bug 990566 - Remove strings made unused by bug 738796. r=dao

browser/locales/en-US/chrome/browser/preferences/preferences.dtd

@@ -24,7 +24,5 @@
 
 <!-- LOCALIZATION NOTE (paneSync.title): This should match syncBrand.shortName.label in ../syncBrand.dtd -->
 <!ENTITY  paneSync.title          "Sync">
-<!ENTITY  buttonForward.tooltip   "Go forward one page">
-<!ENTITY  buttonBack.tooltip      "Go back one page">
 
 <!ENTITY  helpButton.label        "Help">