Well... I have two issues with it that makes me not want to just let it
be...

1) I worry about a future service call not being able to send a router
cable through the system... this will not be a problem if I discontinue
the well and connect the two pipes (the inlet and outlet pipe).

2) I have to pave over that area.

If it's a grease trap, I can't imagine one house producing that much
grease so as to be a problem. If it's some sort of of back-up valve,
the sediment has filled up to the pipes, and so I don't see how it
would accept much backed-up water before sending it on into the house
anyways... however... in case it still has some value as an access
point for roto-cabling further down the line, or as a water-back-up
tank... I might simply take out the tank, and instal a "clean-out" tap
a few feet closer to the house so as to avoid paving over it... the
only question is, I don't know if a clean-out access tap can have a cap
that allows back-up to flow out (sort of like a pressure release
valve), and thus avoid backing up into the house.

Though to be honest, I wish I knew exactly what the thing was.