Remove unused automatic cfg bindings

[brson]

driver.rs has long defined a set of cfg bindings by default, but some of them are no longer used, or have never been used, target_libc, build_compiler, and build_input in particular.

[brson]

Nominating backcompat.

[nickdesaulniers]

@brson , are those three the only ones that can be removed, or are there more? Specifically, which?