There was only one function that required BoringSSL to know how to read
directories. Unfortunately, it does have some callers and it's not immediately
obvious whether the code is unreachable. Rather than worry about that, just
toss it all into decrepit.
In doing so, do away with the Windows and PNaCl codepaths. Only implement
OPENSSL_DIR_CTX on Linux.
Change-Id: Ie64d20254f2f632fadc3f248bbf5a8293ab2b451
Reviewed-on: https://boringssl-review.googlesource.com/7661
Reviewed-by: Adam Langley <agl@google.com>
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