lib/crypto: arm64/chacha: Remove obsolete chunking logic

lib/crypto: arm64/chacha: Remove obsolete chunking logic

Since commit aefbab8e77eb ("arm64: fpsimd: Preserve/restore kernel mode
NEON at context switch"), kernel-mode NEON sections have been
preemptible on arm64.  And since commit 7dadeaa6e851 ("sched: Further
restrict the preemption modes"), voluntary preemption is no longer
supported on arm64 either.  Therefore, there's no longer any need to
limit the length of kernel-mode NEON sections on arm64.

Simplify the ChaCha code accordingly.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20260401000548.133151-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>