Add missing docs to sov-db

[preston-evans98]

Description

Add all missing docs to sov-db and enable the #![deny(missing_docs)] flag. Remove dead code.

Linked Issues

• Related to Cleanup: Add missing docs/tests to existing code #438

[codecov]

Codecov Report

Merging #568 (a116f3b) into nightly (824bfa6) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Files Changed	Coverage Δ
full-node/db/sov-db/src/ledger_db/mod.rs	91.2% <ø> (ø)
full-node/db/sov-db/src/schema/tables.rs	85.0% <ø> (ø)
full-node/db/sov-db/src/schema/types.rs	80.7% <ø> (+1.3%)	⬆️
full-node/db/sov-db/src/state_db.rs	90.6% <ø> (ø)
full-node/db/sov-db/src/lib.rs	100.0% <100.0%> (ø)
full-node/db/sov-db/src/rocks_db_config.rs	95.8% <100.0%> (-0.9%)	⬇️

... and 1 file with indirect coverage changes