Remove WTF::RetainPtr now that it's no longer used

Now that the last conversion from WTF::RetainPtr for the Mac build has
been merged, wtf/retain_ptr.h is no longer used anywhere and we can
safely remove it, so let's do it now to prevent new code from using
WTF::RetainPtr again.

Also, remove an stray include in wtf/text/string_impl_cf.cc that should
have been removed along with CL1518573, when that file was migrated.

Bug: 929973
Change-Id: I5307c1fbf716a4f581fc6b0fadec608f8661bcb2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1520489
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Cr-Commit-Position: refs/heads/master@{#640331}
3 files changed