ArmVirtPkg/QemuFwCfgLib: remove superfluous InternalQemuFwCfgIsAvailable()

ArmVirtPkg/QemuFwCfgLib: remove superfluous InternalQemuFwCfgIsAvailable()

InternalQemuFwCfgIsAvailable() is an API that is incorrectly exposed by
the "OvmfPkg/Include/Library/QemuFwCfgLib.h" library class header; the API
is meant to be used internally to library instances (if it's needed at
all). ArmVirtPkg's instance has no use for it actually, so simplify the
code and remove the function definition.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>