mox-support: remove two firmware files which are not required

These two firmware files were added in this commit 73a39987946b1c54a4c3c4241c0c1dcc42e992e6 ("mox-support: Update WiFi firmware")
without any description. Also, they are not used. I removed it from the
router and it works.

By default, mwifiex and btmrvl are looking for mrvl/sd8997_uapsta.bin
which is wrong as it should look for mrvl/sdsd8997_combo_v4.bin
And because of that, there is a symlink. Pali Rohár sent two fixes to
mailine kernel [1] [2], which fixes it, but it was backported to stable
versions of kernel 5.8, 5.7 and 5.4

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2e1fcac52a9ea
[2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=00eb0cb36fad5

In the future, this is going to be superseded by
https://lists.openwrt.org/pipermail/openwrt-devel/2020-August/030946.html