Drop unnecessary `strict` feature from `lightning-invoice`

Drop unnecessary `strict` feature from `lightning-invoice`

99aa6e27f616c96dda2b49d09bafbc0b982251e0 detected that we had an
undefined feature in `lightning-invoice` called `strict`, which was
used to turn on `deny(warnings)`. It resolved that by adding the
feature to the `Cargo.toml`, but we actually don't need it - our CI
already builds with `-Dwarnings`, so any warnings should be
rejected during CI and there's not a lot of value in having a
(public) feature to do the same.