Remove unnecessary librust-clap-2 feature Build-Depends

Jeremy Bícharequested to merge
wip/clap-bd into debian/latest

Debian's rust-clap-2 package no longer builds the ansi-term feature. Since Cargo.toml doesn't specify any clap-2 features, only use Build-Depends: librust-clap-2-dev instead of various features.

This fixes the build. I didn't try running elan though.

cc/ @mans0954

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