Remove Blockstore fifo compaction code

[steviez]

Problem

Use of fifo compaction (via --rocksdb-shred-compaction fifo) has been deprecated.

• Deprecate --rocksdb-shred-compaction fifo #1882 initially made intent to deprecate known, and has more context and links for the "why"
• Disallow --rocksdb-shred-compaction fifo in the validator #3451 made it invalid to specify the value fifo at the CLI level
• Even though fifo can't be set, the code to handle fifo is still presently in the codebase

Summary of Changes

Since fifo is no longer a valid option, we can remove all of the code that configures it / handles choosing between level and fifo (since level is the only choice now)