LoongArch: Remove redundant "source drivers/firmware/Kconfig"

In drivers/Kconfig, drivers/firmware/Kconfig is sourced for all ports so
there is no need to source it in the port-specific Kconfig file.  And
sourcing it here also caused the "Firmware Drivers" menu appeared two
times: one in the "Device Drivers" menu, another in the toplevel menu.
This is really puzzling so remove it.

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>