ArmVirtPkg/ArmVirtXen: don't set PcdShellFile

ArmVirtPkg/ArmVirtXen: don't set PcdShellFile

According to the

  PCDs not used by modules or in conditional directives

sections of all the build reports for

  {AARCH64,ARM} x {Xen} x {DEBUG,NOOPT,RELEASE} x {feat-0,feat-1}

(12 builds in total), the PCD is not used in any of those builds. Remove
the setting.

("feat-1" stands for "-D HTTP_BOOT_ENABLE -D NETWORK_IP6_ENABLE -D
SECURE_BOOT_ENABLE -D TTY_TERMINAL", while "feat-0" stands for "".)

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Julien Grall <julien.grall@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>