Remove FLATC_EXECUTABLE and the ability to bring your own flatc

[jathu]

Summary

• Remove the ability to set flatc using FLATC_EXECUTABLE and always build it from source
  • Also remove, the now redundant, EXECUTORCH_BUILD_FLATC
• Move the flatc configuration into third-party/CMakeLists.txt to keep things organized
• Allow FLATBUFFERS_MAX_ALIGNMENT to be configurable

Test plan

CI

cc @larryliu0820

[pytorch-bot]

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❌ 1 New Failure, 2 Pending

As of commit ce4507c with merge base 6e959be ():

NEW FAILURE - The following job has failed:

• trunk / test-arm-backend (test_pytest_ops_ethosu_fvp) / linux-job (gh)
  RuntimeError: Command docker exec -t de8e7321e4594cb8c693f9f0424db5a6bdb82663b218876acabfcd77066ab1ad /exec failed with exit code 1

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[larryliu0820]

Why do we need this

[jathu]

I want to stop directly reaching into flatbuffers, and instead use the flatbuffer library target:

executorch/runtime/executor/test/CMakeLists.txt

Line 158 in bf50527

"${EXECUTORCH_ROOT}/third-party/flatbuffers/include"

I'll do it in some future diff