ramips: remove non-existant poe_passthrough for RouterBOARD 750Gr3

ramips: remove non-existant poe_passthrough for RouterBOARD 750Gr3

This 750gr3 GPIO17 switch was added based on vendor source,
but only the 760iGS (which shares the rbsysfs board identifier)
device has the physical wiring. The 750Gr3 actually does not
support PoE out.

Apart from that, note that the gpio base (480) would have required
this GPIO to be referenced as 497 if it was kept.

Fixes: 6ba58b7b020c ("ramips: cleanup the RB750Gr3 support")
Signed-off-by: John Thomson <git@johnthomson.fastmail.com.au>
[commit title/message facelift]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>