Use native check-cfg lint in cargo beta

Use native check-cfg lint in cargo beta

This uses the newly introduced conditional configuration checks that are
now configurable withint Cargo (beta).

This allows us to get rid of our custom python script that checks for
expected features and cfgs.

This does introduce a warning regarding the unknown lint in Cargo
versions prior to the current beta, but since these are not rustc errors,
they won't break any builds with the "-D warnings" RUSTFLAG.

Moving to this lint actually exposed the "strict" feature not being
present in the lightning-invoice crate, as our python script didnt
correctly parse the cfg_attr where it appeared.