These static output buffers are a legacy from a time before processes
had threads. This change drops support and callers who were depending on
this (of which there are hopefully none) will crash.

Change-Id: I7b8eb3440def507f92543e55465f821dfa02c7da
Reviewed-on: https://boringssl-review.googlesource.com/14528
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>