Remove LRU block cache

**Is your feature request related to a problem? Please describe.**
In #4920 the LRU block cache was deprecated and removed as the default and TinyLFU was set as the new deault.

**Describe the solution you'd like**
Remove the LRU block cache code if the TinyLFU default works well in all 4.0.0 testing.

The LRU code could be left as is. However its i a large amount of code and since its no longer the default it will not be exercised as much and therefore the confidence in its correctness will decrease over time as it is used less and the codebase changes around it. So it would be good to remove it once the new default has been well exercised because its doubtful anyone will feel comfortable recommending its use.

Start by reviewing issue #4920 and the 4.0.0 testing that exercises the TinyLFU default. Locate the LRU block cache implementation and its related references, then confirm the full test suite passes without it. Done means the obsolete LRU code and references are removed while TinyLFU remains the working default.

Written by the indexing model from the issue text.