block-cache-size is deprecated since v6.6.0

[choudharypankaj]

File: /release-8.1/tune-tikv-memory-performance.md

It looks like the block-cache-size is deprecated since v6.6.0.
https://docs.pingcap.com/tidb/stable/tikv-configuration-file#block-cache-size

Can we remove the config listed on the document:
`## Size of the shared block cache. Normally it should be tuned to 30%-50% of system's total memory.

When the config is not set, it is decided by the sum of the following fields or their default

value:

* rocksdb.defaultcf.block-cache-size or 25% of system's total memory

* rocksdb.writecf.block-cache-size or 15% of system's total memory

* rocksdb.lockcf.block-cache-size or 2% of system's total memory

* raftdb.defaultcf.block-cache-size or 2% of system's total memory`