rtl8188fu-openipc: drop unused firmware blob (driver embeds it)

[widgetii]

Summary

Follow-up cleanup to #2157, per @flyrouter's catch in #2157 (comment).

The openipc/realtek-wlan rtl8188fu branch already carries the firmware compiled into the .ko:

• include/autoconf.h defines LOAD_FW_HEADER_FROM_DRIVER unconditionally
• hal/rtl8188f/hal8188f_fw.c (~372 KB source) emits u8 array_mp_8188f_fw_*[] byte arrays that the driver consumes directly — no request_firmware() call

So the rtl8188fufw.bin file I shipped to /lib/firmware/rtlwifi/ in #2157 was dead weight on every board that builds 8188fu.ko. This PR drops:

1. general/package/rtl8188fu-openipc/rtl8188fufw.bin (the orphaned blob, ~21 KB)
2. The RTL8188FU_OPENIPC_INSTALL_FIRMWARE POST_INSTALL hook + the proprietary (rtl8188fufw.bin firmware blob) LICENSE note in rtl8188fu-openipc.mk
3. BR2_PACKAGE_LINUX_FIRMWARE_OPENIPC_RTL_8188EU=y from gk7202v300_ultimate_defconfig — I'd added this in gk7202v300: add ultimate variant + ship rtl8188fufw blob from the FU package #2157 by mistake; the gk7205v200_ultimate template never had it, and r8188eu from the kernel staging tree on this platform doesn't load rtl8188eufw.bin either

Local verification (make BOARD=gk7202v300_ultimate)

- uImage:         1890 KB / 2048 KB   (unchanged)
- rootfs.squashfs:7040 KB / 8192 KB   (was 7064 KB, -24 KB)

unsquashfs -l rootfs.squashfs.gk7202v300 | grep -E 'rtl8188|8188fu|rtlwifi|wpa_sup':

squashfs-root/etc/wpa_supplicant.conf
squashfs-root/lib/modules/4.9.37/extra/8188fu.ko
squashfs-root/lib/modules/4.9.37/kernel/drivers/staging/rtl8188eu/r8188eu.ko
squashfs-root/usr/sbin/wpa_supplicant

/lib/firmware/rtlwifi/ is gone (no consumers). FU and EU kernel modules both still ship — only the unused blobs leave.

Test plan

• CI matrix builds gk7202v300_ultimate cleanly
• CI matrix hi3518ev200_ultimate continues to pass (regression check — that board's image also stops shipping the dead blob)
• On-device: same flash recipe (fw_setenv wlandev rtl8188fu-generic → reboot → wlan0 up) should still work, because the FU driver's firmware is in the .ko

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