[][openwrt][mt7988][tops][remove script 98-mtk-vpn]

[Description]
Remove script 98-mtk-vpn from common part. Add it to platform-specific
base-files. This script disable HNAT once PPTP connection is
established, preventing PPE from mis-binding of PPTP traffic. This
problem exits only on Netsys v2 hardware (MT7986, MT7981), and no longer
exists on Netsys V3 hardware (MT7988). Besides, TOPS PPTP offload would
not work if this script is not removed.

[Release-log]
N/A

Change-Id: Ia6f82c7c5ccdf21020cad110b03e2a8a49ab88d5
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Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8385228
3 files changed