Revert "Added Abseil as a third-party dependency"

This reverts commit 020d69905e.

Reason for revert: Causing Dawn Skia roll build fails

https://task-scheduler.skia.org/job/AZXXBm9h9ozj3kKiRJM5

Original change's description:
> Added Abseil as a third-party dependency
>
> Using the version in the Chromium repo in order to make use of the .gn
> files it contains.
>
> Doing so also appears to require us to switch where we pull googletest
> from so that the directory structure matches what the Abseil build
> config expects. Fortunately this doesn't seem to cause issues in our
> tests.
>
> Bug: dawn:563
> Change-Id: I55831ad33f282b3d8b03b67826fd2776e5602d89
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/63780
> Reviewed-by: Austin Eng <enga@chromium.org>
> Reviewed-by: Corentin Wallez <cwallez@chromium.org>
> Commit-Queue: Corentin Wallez <cwallez@chromium.org>
> Auto-Submit: Brandon Jones <bajones@chromium.org>

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: dawn:563
Change-Id: I0fc4e5fc73ab9b0887591135ec01adde990edd6f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/64361
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Shrek Shao <shrekshao@google.com>

.gitignore

@@ -7,7 +7,6 @@
 build
 buildtools
 testing
-third_party/abseil-cpp/
 third_party/angle/
 third_party/clang-format/
 third_party/glfw/