mac: Remove libc++ hack that was required by NaCl.

NaCl target were still built with the 10.6 SDK even after we thought we
switched the whole build to the 10.10 SDK. This was a problem in static
library ASan builds: -fsanitize=address implicitly requires libc++, so
this added a -lc++ to the link, but the 10.6 SDK doesn't contain a libc++
(since ASan requires 10.7, and since it also adds an implicit -lc++abi,
we dynamically link libc++ in ASan builds). This led to linker errors.

Now that NaCl targets build with the 10.10 SDK like everything else when
they're part of a Chromium build, the hack I used to work around this is
no longer needed, so remove it.

No intended behavior change.

BUG=400091

Review URL: https://codereview.chromium.org/1414703004

Cr-Original-Commit-Position: refs/heads/master@{#356325}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e1c27eab364747ef85275615835bf56514916696