Mk/Uses/cargo.mk: Do not override do-configure

Cargo vendoring can be useful even for ports that use other build
systems like Meson to call Cargo for the actual build. Often they
use the framework's default do-configure which USES=cargo should
not overwrite.

Remove hacks from games/jaggedalliance2 and x11/squeekboard.

Reviewed by: jbeich
Differential Revision: https://reviews.freebsd.org/D22619