Remove unused `LoggableBatch::num_instances` #12494

Job	Run time
cpp-paths-filter	6s
docs-paths-filter	5s
python-paths-filter	8s
rust-paths-filter	5s
web-paths-filter	5s
Checks / Check if running codegen would produce any changes	1m 14s
Checks / Test Python Docs	24s
Checks / Lint markdown	17s
Checks / Rerun lints	31s
Checks / Misc formatting	18s
Checks / Spell Check	8s
Checks / Check links	15s
Update PR Body / Update PR body	17s
Deploy Landing Preview	0s
Build rerun_js / Build rerun_js	41s
Build web viewer / Build web viewer	33s
Rust Checks / Rust lints (fmt, check, clippy, tests, doc)	32s
Rust Checks / Test on macOS and Windows	0s
C++ tests	0s
Python Checks	0s
Minimum CLI Build / Set Config (linux-x64)	2s
Minimum CLI Build / Build rerun-cli (ubuntu-latest-16-cores)	3m 38s
Upload Web	0s
Save PR Summary	0s
Minimum Wheel Build / Set Config (linux-x64)	3s
Minimum Wheel Build / Build Wheels (ubuntu-latest-16-cores)	1m 56s
Minimum Wheel Test / Set Config (linux-x64)	1s
Minimum Wheel Test / Test Wheels (ubuntu-latest-16-cores)	4m 20s
	15m 39s

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