ArmPlatformPkg: remove ArmVExpress-CTA15-A7.dsc/fdf

ArmPlatformPkg: remove ArmVExpress-CTA15-A7.dsc/fdf

ARM Ltd Platform support is migrating to use OpenPlatformPkg [1].

Currently, TC2 exists both in EDK2's ArmPlatformPkg and in
OpenPlatformPkg [2].  And they are starting to diverge, with
OpenPlatformPkg being the most up-to-date with current developments.
To prevent this divergence, remove the .dsc and .fdf files from
ArmPlatformPkg and leave OpenPlatformPkg as the master.

[1] https://git.linaro.org/uefi/OpenPlatformPkg.git

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>