Delete `ParsedPartitionSpec` and `preprocess` function and do a couple more cleanups #5768

Job	Run time
build-jax-artifacts (jaxlib) / Build jaxlib (linux-x86-n2-16, Python 3.10, clone main XLA=1)	2m 23s
build-jax-artifacts (jax) / Build jax (linux-x86-n2-16, Python 3.10, clone main XLA=1)	1m 40s
TPU test (jaxlib=head, v5e-8) / Pytest TPU (v5e-8, Python 3.10, libtpu=nightly)	6m 26s
	10m 29s

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