Make sure about:newtab is blank

My understanding is that currently the desired behaviour is that about:newtab pages should be blank instead of the default Firefox newtab page. While working on #30429 (moved) I realized that after applying most of the desktop patches about:newtab is still the Firefox default. Setting browser.newtabpage.enabled = false achieves the desired effect, but this pref is true for current esr60 and I could not find a browser patch that explicitly sets about:newtab to blank for current esr60.

In any case, we should probably set browser.newtabpage.enabled = false for esr68.