Remove deprecated functionality

[EwoutH]

Remove some deprecated functionality that is scheduled to be removed with 3.1.

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 -4.2% [-5.7%, -2.8%]	🔵 -0.4% [-0.7%, -0.2%]
BoltzmannWealth	large	🔵 -0.5% [-1.0%, -0.0%]	🔵 -1.3% [-1.6%, -1.1%]
Schelling	small	🔵 -0.9% [-1.2%, -0.6%]	🔵 -1.6% [-1.8%, -1.3%]
Schelling	large	🔵 -1.1% [-1.4%, -0.9%]	🔵 +0.6% [-0.6%, +1.9%]
WolfSheep	small	🔵 +0.1% [-0.1%, +0.3%]	🔵 -1.0% [-1.1%, -0.8%]
WolfSheep	large	🔵 -0.7% [-1.3%, -0.1%]	🔵 +1.2% [+0.2%, +2.6%]
BoidFlockers	small	🔵 +1.0% [+0.3%, +1.7%]	🔵 +1.0% [-0.2%, +2.2%]
BoidFlockers	large	🔵 +2.3% [+1.8%, +2.7%]	🔵 +0.6% [-0.3%, +1.5%]