font whitelist means we don't have to set gfx.downloadable_fonts.fallback_delay

In 8455, "gfx.downloadable_fonts.fallback_delay" was set to -1 to avoid temporarily rendering a local font, which would allow its characters to be measured. But now that we whitelist fonts, it is probably OK to stop setting this pref. We should confirm that the fallback mechanism doesn't provide a whitelist bypass.