Remove impl_align_conversions

[nicoburns]

These lines:

• Were introduced in the "Commit all other changes from Servo" commit (I wont put a git commit id due to the frequent rebasing in this repo)
• Are #[cfg(feature = "gecko")]
• Are not present in upstream Stylo

I therefore conclude that we can safely remove them from our downstream copy.

I believe commit could even be squashed into the "Commit all other changes from Servo" commit next time main is being rebased (which would simplify the history as the change would disappear entirely rather than being introduced and then removed again).