ArmPlatformPkg/ArmJunoPkg: remove PCI host bridge driver

We are switching the Juno platform to the generic host bridge driver,
which involves implementing PciHostBridgeLib for this platform, and
plugging it into MdeModulePkg's PciHostBridgeDxe.inf.

Since the platform descriptions no longer live in upstream EDK2, the
PciHostBridgeLib implementation (which reuses some of the code removed
here) will live there as well. But this PciHostBridgeDxe driver is no
longer used, so remove it.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>