Remove constructor with preFill argument from Pool

[obigu]

Unfortunately I've realised that the constructor based mechanisms that #5773 introduced to prefill a Pool is bad and should be removed (we can keep calling fill() after instantiation).

Calling a protected method (newObject()) on the constructor is an anti pattern and can cause state consistency issues as the object may not have been fully initialized. A simple example can be seen on ParticleEffectPool in which if we use the constructor passing preFill() as true, newObject() will be called before effecthas been set throwing a NullPointerException.

I should have seen this issue before, sorry for that :(

[NathanSweet]

Can you add a line to CHANGES?

[obigu]

@NathanSweet Done, thanks