Remove EHRPD DUN apn.

EHRPD is not a single-Dc bearer as defined by
config_onlySingleDcAllowed (and noticed in practice). By
duplicating the APNs we actually connect twice and bad things
happen (tethering doesn't work).

bug:17933212
Change-Id: Id76073130d468019b56d69e531652e99c745f69e