SPDK supported max ios per sec for read/write combined, so we added
support for it. However, cgroup v2 (which we’re migrating to for I/O
rate limiting) does not support combined IOPS limits — only separate
`IOReadIOPSMax` and `IOWriteIOPSMax`.
This patch removes the unused `max_ios_per_sec` knob. It was never used
in production, and our current rate limiting relies solely on read/write
bandwidth, which is fully supported by cgroup v2.
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