cmd: qfw: remove qemu_fwcfg_free_files()

This patch is part of the qfw refactor work.

The qemu_fwcfg_free_files() function is only used in error handling in
ACPI table generation, let's not make this a core function and move it
to the right place.

Signed-off-by: Miao Yan <yanmiaobest@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>