Update the .gn to remove forcing use_custom_libcxx = false. Instead, let
//build/config/c++/c++.gni determine the value, which should better
match Chromium.

In particular, this helps with Abseil integration by supporting Windows
component builds by better matching the contents of the .def files.

Bug: angleproject:4873
Test: CQ
Change-Id: If8c064f16459292bfb95113a4d78e14e8923f86f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2414739Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>