mac: remove libc++-static 10.6 compatibility hack

This hack is obsolete since the minimum supported Mac version is now 10.9.

This CL also removes the checked-in copy of libc++-static, and removes
it from DEPS.

BUG=

Review-Url: https://codereview.chromium.org/2314753002
Cr-Original-Commit-Position: refs/heads/master@{#418584}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9fe8c63c54c0a0c4fd420518962e462cc00f9f65