OvmfPkg/XenAcpiPlatformDxe: remove QEMU fw_cfg dependency

OvmfPkg/XenAcpiPlatformDxe: remove QEMU fw_cfg dependency

The QemuDetected() function wraps QemuFwCfgIsAvailable(); it always fails
on Xen. Because of that, we can eliminate the QemuDetected() call itself
from the Xen ACPI platform driver, and then the rest of "Qemu.c" becomes
useless -- the workhorse function of that source file is
QemuInstallAcpiTable(), which we no longer call.

Remove any dependencies that are no longer needed by the
XenAcpiPlatformDxe driver.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien@xen.org>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210526201446.12554-15-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>