Rm platform args in executorch

[jaejunku]

Summary:
Get rid of platform args around the repo to unblock modernization with platforms.
Platform args were only needed in buck1

Differential Revision: D88612868

[pytorch-bot]

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 4e2f6df with merge base 9156fff ():

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)
  Timeout waiting for emulator to boot.

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[meta-codesync]

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