Unify the two BoringSSL codepaths a bit and simplify init

Although bssl-sys (currently) provides init, it only does so for
rust-openssl, and it's more natural for rust-openssl to consistently be
responsible for it. Also BoringSSL has not required initialization at
all since June 2024.

I've also removed the ambiguous_glob_reexports allowance. I couldn't
reproduce whatever triggered it, and it seems unlikely that the two
codepaths would have different needs here.

This leaves the remaining non-build unstable_boringssl trigger so minor
that perhaps we should just remove it here? It dates to some problematic
Android Rust integration. At this point, it's just an external mechanism
to get the same output as the build.rs logic around bindgen, mostly
because bindgen has perpetual deficiencies w.r.t.  correctly binding C,
so we need to work around it. Ideally there wouldn't be two sources of
those workarounds, but Google's build environments care about build
reproducibility and hermeticity, while Rust and Cargo have... very much
not achieved this yet. But there's no reason for rust-openssl's build to
care about this.