revert(perf): remove Flerovium and the spawn tuning

Run 3 benches (analysis/client-perf/RESULTS-proxy.md): Flerovium's run-2
win did not reproduce (S1 slightly worse, S3 within noise), and the
moderate spawn tuning (d80f85b, 8e8483d) left total mob counts unchanged
across 4 runs per config. Owner approved removing both. The mods now
regenerate their default spawn configs.

mods/flerovium.pw.toml

@@ -1,13 +0,0 @@
-name = "Flerovium"
-filename = "flerovium-neoforge-1.21.1-1.1.2-all.jar"
-side = "client"
-
-[download]
-url = "https://cdn.modrinth.com/data/4Rh1Mobu/versions/oU1q3eIj/flerovium-neoforge-1.21.1-1.1.2-all.jar"
-hash-format = "sha512"
-hash = "6301ea6991768832fa983f61e1b0574336aea4920e027632a3df93c8a10f5d87041876fad516d16827d9cd13a84b5a8c334c1a0076ca04121e321939d5d6ea52"
-
-[update]
-[update.modrinth]
-mod-id = "4Rh1Mobu"
-version = "oU1q3eIj"