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This hack is obsolete since the minimum supported Mac version is now 10.9.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-Commit-Position: refs/heads/master@{#418584}
8 files changed