hw/nvram/fw_cfg: Remove the unnecessary boot_splash_filedata_size

The 'boot_splash_filedata_size' was introduced as a global variable
in 3d3b8303. This variable is used as a 'size' argument to the
fw_cfg_add_file(). This function has an interface contract with its
'data' argument, but there is no such contract for 'size' (this is
not a referenced pointer).  We can simply remove it.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190308013222.12524-7-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>