Delete all in-tree benchmark infrastructure code. (iree-org#18144)

See the related RFC:
https://groups.google.com/g/iree-discuss/c/n039Q0trkHk

Deleted components _directly_ related to benchmark infrastructure:
* Workflow files in `.github/workflows/`: benchmark triggering, e2e test
artifact generation, benchmark compilation stats, benchmark execution,
comment posting
* Benchmark configuration and series definitions (mix of Python scripts,
CMake generators, CMake files, etc.)
* Documentation relating to benchmarks on pull requests / using GitHub
Actions

Deleted components _indirectly_ related to benchmark infrastructure:
* Support scripts like
`build_tools/benchmarks/set_android_scaling_governor.sh` (could be
useful for developers, but doesn't need to live in-tree)
* `iree_run_module_test` (was used to test correctness for _some_
benchmark workloads but was not fully adopted)
* Build presets in `build_tools/cmake/` scripts (e.g. `test`,
`benchmark`, `benchmark-with-tracing`): too complex, trying to steer
away from build scripts and towards CMake presets or explicit workflows
without that indirection

Future cleanup possibilities:
* `build_all.yml` (no longer any references)
* Docker files: android, nvidia, nvidia-bleeding-edge

Closes iree-org#7795
Closes iree-org#8631
Closes iree-org#10599
Closes iree-org#10928
Closes iree-org#11703
Closes iree-org#11922
Closes iree-org#11953
Closes iree-org#14993
Closes iree-org#15452
Closes iree-org#16057
Closes iree-org#16157
Closes iree-org#16396
Closes iree-org#16431
Closes iree-org#17749

.github/CODEOWNERS

@@ -38,7 +38,6 @@
 
 # Infra Top-Level Directories
 /build_tools/ @ScottTodd
-/build_tools/benchmarks/ @antiagainst
 /build_tools/python_deploy/ @stellaraccident
 /build_tools/scripts/ @ScottTodd
 /build_tools/third_party/ @ScottTodd @stellaraccident