ruby: remove rubygems

It's currently in a non-working state since it requires a couple of
extensions that don't build at the moment (they try to execute tests in
configure) and also requires a target compiler.
So remove it to avoid false expectations and reclaim target space back
of about 1.5 MiB.

[Thomas: slightly reword comments.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>