[mlir][acc] Erase empty kernel_environment ops during canonicalization (#166633)

This change removes empty `acc.kernel_environment` operations during
canonicalization. This could happen when the acc compute construct
inside the `acc.kernel_environment` is optimized away in cases such as
when only private variables are being written to in the loop.

In cases of empty `acc.kernel_environment` ops with waitOperands, we
still remove the empty `acc.kernel_environment`, but also create an
`acc.wait` operation to take those wait operands to preserve
synchronization behavior.

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