Remove unnecessary deps on libc++

Also:
* Add no_default_deps to mini_installer.
* Updates an incorrect comment and removes an unnecessary dependency on
  libc++ in mini_installer/BUILD.gn which became broken in [1].

[1] https://chromium-review.googlesource.com/1073613

BUG=None

Change-Id: I44f8f09e36f2764b3077af5d0cd7605a575b347e
Reviewed-on: https://chromium-review.googlesource.com/1157509
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579879}
2 files changed