ath79: mikrotik: drop unused files from ramdisk

ath79: mikrotik: drop unused files from ramdisk

The ramdisk used by sysupgrade on MikroTik devices currently includes
U-Boot fw_* files that are not necessary for performing a system upgrade
on that platform.  The relevant lines were added to
target/linux/ath79/mikrotik/base-files/lib/upgrade/platform.sh by commit
a66eee6336 ("ath79: add mikrotik subtarget"), likely because they also
existed in target/linux/ath79/nand/base-files/lib/upgrade/platform.sh,
where the platform_do_upgrade_mikrotik_nand() function moved by commit
a66eee6336 originally lived.  However, these lines were added to
target/linux/ath79/nand/base-files/lib/upgrade/platform.sh by commit
55e6c903ae ("ath79: GL-AR300M: provide NAND support; increase to 4 MB
kernel"), which is not related to MikroTik devices in any way.

Remove the code adding unused U-Boot fw_* files to the ramdisk used by
sysupgrade on MikroTik devices.

Signed-off-by: Michał Kępień <openwrt@kempniu.pl>