ramips: remove further mt7621 code from local ethernet driver

ramips: remove further mt7621 code from local ethernet driver

While commit 734a8c46e703 focussed on removing stuff directly
selected by the NET_RALINK_* symbols, this patch removes additional
unused mt7621-specific code from the ethernet driver.

As with the previous patch, the main reason is to reduce the amount
of code we have to maintain and care about.

Note that this patch still keeps a few lines with
IS_ENABLED(CONFIG_SOC_MT7621) in mtk_eth_soc.h/.c, as this file is
still selected for the mt7621 subtarget.

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