Remove mipsel target_arches

[xen0n]

The mipsel target_arch was introduced with the initial MIPSel support (rust-lang/rust@82ec1ae), but was subsequently removed with implementation of the Flexible Target Specification (Rust RFC 0131, rust-lang/rust@3a8f4ec). Remove these to be consistent with other arches and reduce confusion.

I've very slightly formatted the deletions so as to not require a style cleanup later and obscure git blame output.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[xen0n]

(I was confused by this myself, and this is why I incorrectly done #443 when, in fact, the problem was that [replace]s inside sub-Cargo.tomls (instead of the root-level one) are ignored, not picking up my changes.)

[alexcrichton]

Thanks!