Remove unused `Datatype` and `DatatypeBatch` #12487

Job	Run time
cpp-paths-filter	6s
docs-paths-filter	8s
python-paths-filter	4s
rust-paths-filter	5s
web-paths-filter	3s
Checks / Test Python Docs	24s
Checks / Check if running codegen would produce any changes	1m 12s
Checks / Lint markdown	17s
Checks / Rerun lints	25s
Checks / Misc formatting	21s
Checks / Spell Check	8s
Checks / Check links	22s
Update PR Body / Update PR body	23s
Python Checks	0s
Build rerun_js / Build rerun_js	3m 50s
Build web viewer / Build web viewer	3m 16s
Rust Checks / Test on macOS and Windows	0s
Minimum CLI Build / Set Config (linux-x64)	3s
Rust Checks / Rust lints (fmt, check, clippy, tests, doc)	12m 16s
Deploy Landing Preview	0s
C++ tests	0s
Minimum CLI Build / Build rerun-cli (ubuntu-latest-16-cores)	4m 56s
Upload Web / Upload web build to google cloud (wasm32 + wasm-bindgen)	14s
Save PR Summary / Create HTML summary for PR	41s
Minimum Wheel Build / Set Config (linux-x64)	1s
Minimum Wheel Build / Build Wheels (ubuntu-latest-16-cores)	2m 20s
Minimum Wheel Test / Set Config (linux-x64)	2s
Minimum Wheel Test / Test Wheels (ubuntu-latest-16-cores)	4m 11s
	35m 48s

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