Rollup merge of rust-lang#24751 - lstat:feature-gate-22820-dups, r=brson

As part of the audit for rust-lang#22820 the following duplicate feature
gate tests were removed:

* `box_patterns`
* `simd_ffi`

These tests for `box_patterns` and `simd_ffi` were added in rust-lang#23578,
however there were existing tests in rust-lang#20723 and rust-lang#21233 respectively.

r? @nrc

Author: steveklabnik