Remove RAM accounting from LeafReader

Context for this issue can be found at https://lists.apache.org/thread.html/r06b6a63d8689778bbc2736ec7e4e39bf89ae6973c19f2ec6247690fd%40%3Cdev.lucene.apache.org%3E.

RAM accounting made sense when readers used lots of memory. E.g. when norms were on heap, we could return memory usage of the norms array and memory estimates would be very close to actual memory usage.

However nowadays, readers consume very little memory, so RAM accounting has become less valuable. Furthermore providing good estimates has become incredibly complex as we can no longer focus on a couple main contributors to memory usage, but would need to start considering things that we historically ignored, such as field infos, segment infos, NIOFS buffers, etc.

Let's remove RAM accounting from LeafReader?