ramips: mt7621: include uboot-envtools in some missed devices

ramips: mt7621: include uboot-envtools in some missed devices

Add uboot-envtools (removing -uboot-envtools) to devices which were
missed in the commit 79bd017 ("ramips: mt7621: add uboot-envtools to
all devices")

- Mi Router 3G
- Mi Router AC2100

While at here remove two redundant entries from devices which were added
just after the referenced commit 79bd017 and did not account for the
new DEFAULT_PACKAGES member:

- SNR-CPE-ME1
- SNR-CPE-ME2-SFP

Fixes: 79bd017 ("ramips: mt7621: add uboot-envtools to all devices")
Fixes: https://github.com/openwrt/openwrt/issues/18960
Signed-off-by: Mario Andrés Pérez <mapb_@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/19012
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>