hw/arm/virt: Remove null-check in virt_build_smbios()

hw/arm/virt: Remove null-check in virt_build_smbios()

Since commit 578f3c7b0835 ("arm: add fw_cfg to "virt" board",
2014-12-22), the machvirt_init() unconditionally creates the
fw_cfg object.  Later, commit c30e15658b1b ("smbios: implement
smbios support for mach-virt", 2015-09-07) added a superfluous
null-check on it.
Remove this superfluous check.

Reviewed-by: Laszlo Ersek <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20190309181920[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>