This reverts commit 1acaf4ec.

Reason for revert: Fails when is_component_build is not set:

lld-link: error: <root>: undefined symbol: public: __cdecl absl::Condition::Condition<struct std::__1::atomic<bool> const>(bool (__cdecl *)(struct std::__1::atomic<bool> const *), struct std::__1::atomic<bool> const *)
<snip>

Original change's description:
> Convert unordered_map to absl::flat_hash_map for select files
> 
> This is the initial CL to start migrating to abseil in various places:
> - formatutils.h
> - FramebufferVk.h
> - Program.h
> - ProgramExecutableVk.h
> - RewriteRowMajorMatrices.cpp
> 
> This intentionally hits a couple different places in the code to make
> sure the abseil dependencies are added to the required targets.
> 
> Bug: angleproject:4873
> Change-Id: I68c7d067b6912b0cc0ecde231501dbed92f0b189
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2321735
> Commit-Queue: Tim Van Patten <timvp@google.com>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Reviewed-by: Charlie Lao <cclao@google.com>

TBR=timvp@google.com,jmadill@chromium.org,cclao@google.com

Change-Id: I75041532fc5126b4c7cc5e0d4529883fb357e05b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: angleproject:4873
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2383870Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>