ramips: remove legacy ethernet driver components for mt7621

ramips: remove legacy ethernet driver components for mt7621

The mt7621 subtarget has been switched to DSA quite a while ago and
seems to run sufficiently fine. Build with older kernels than 5.4 has
been disabled directly during the kernel bump, so our local ethernet
driver is unused in master since then.

Therefore, let's remove the mt7621-specific parts of "our" ethernet
driver, so we don't have to maintain them and it's obvious to
everybody that they are not used anymore.

This also drops the offloading components as this was specifically
implemented to depend on mt7621.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>