Remove crypto/curve25519.h in favor of BoringSSL's.

They're the exact same functions, only with BoringSSL/C-style
argument order rather than Chromium/C++-style. (Outputs first
rather than last.) The //crypto version is just a wrapper.

BUG=none

Review-Url: https://codereview.chromium.org/2457153003
Cr-Commit-Position: refs/heads/master@{#447090}
8 files changed