dev-arm, configs: Remove RealViewPBX platform

dev-arm, configs: Remove RealViewPBX platform

This is an old unused platform. We should support VExpress_GEM5 based
platforms only.

Change-Id: If9c29047b2d068992dfbbe0dc268c70b788cce5f
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21601
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>