Stop using Mozilla user agent header

Mozilla user agent header was initally used
because some sites required that a valid
browser be used to load; however, for other
sites this resulted in connection errors.

This a temporary solution; in the future,
we may try multiple request types on upon
failure.

Bug: T1174
Bug: T1173
Change-Id: I9e0cc7e1bb359cd4610048b85a7c62a77d2d2127