Remove low-signal FileDataLoader allocation-failure tests

[metascroy]

Summary:
D109072812 added two allocation-failure unit tests to
file_data_loader_test.cpp -- AllocationFailureDuringLoadReturnsError and
AllocationFailureDuringFromReturnsError -- driven by a strong global
replacement of operator new(size_t, std::align_val_t, const std::nothrow_t&)
that returns nullptr on demand.

That override broke the fbcode build (see T276843331): fbcode test binaries link
jemalloc, which provides its own strong definition of the same operator-new
symbol, so the second strong definition produces an ld.lld: error: duplicate symbol link failure (it likewise clashes with the ASAN allocator runtime). The
original diff only validated the xplat/Apple target, where libc++ provides the
operator-new family as weak symbols, so the breakage was not caught.

These tests are also low signal: they only exercise the pre-existing
nullptr -> Error::MemoryAllocationFailed guard, and because they replace the
allocator wholesale they never actually exercise the production fix from
D109072812 (switching et_aligned_alloc to the nothrow operator new).
Covering that trivial guard is not worth a permanent global-operator-new
override and its cross-platform build fragility.

This change removes both tests and the operator-new override, reverting only the
test-file additions from D109072812. The production fix from that diff -- using
the nothrow operator new in et_aligned_alloc -- is retained. Applied to both
the fbcode and xplat mirror copies of file_data_loader_test.cpp.

Differential Revision: D109506462

[pytorch-bot]

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❌ 3 New Failures, 3 Unrelated Failures

As of commit 4619249 with merge base aada6d7 ():

NEW FAILURES - The following jobs have failed:

• pull / test-arm-backend-no-driver (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t c1ad0750975214fc4715dfaf503006bc506c23b3344753f0372353019cff2dcb /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 060fb177c1d4adc42fbd6e7dcfdcb13830f9e7c05f5c0bc3c39d13b830b99d7e /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 31ab3e9af2578e5c5af20bd49e0da071f2374e8b7040efb32baaaddcc26b1fb3 /exec failed with exit code 1

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)
• pull / unittest / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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[meta-codesync]

@metascroy has exported this pull request. If you are a Meta employee, you can view the originating Diff in D109506462.

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