Remove LRU block cache

[keith-turner]

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.

[Amemeda]

Looking into this, what is the status of 4.0.0 testing? Is it safe to remove LRU block cache now?

[ddanielr]

I'm not aware of any issues with the block cache when testing with 4.0.0.

Users are able to change the block cache class via a property since version 2.0.0. so they do have a chance to test TinyLFU today without needing to go to 4.0.0

[ctubbsii]

I'm not too worried about removing it now. I haven't seen any issues with the default. But, if we do find issues, it's trivial to bring it back or provide an alternate implementation in a supplemental jar to add to the classpath.