Remove cache of slots per class in FMRelationSlot

[NicolasAnquetil]

I found a case (PowerBuilder) were this cache was causing bugs
Is it really needed ?

[jecisc]

I've added the cache because it cause a great performance regression on MooseQuery on big models. The introduction of the cache improved a lot the performances of Benoit's tooling

[badetitou]

Yep. The cache is super important in my tool.

Is it creates bug, I know that before there was a strategy pattern that allows us to use or not cache

We can do it again maybe?
By default use cache ( it is nice for user imo ). And one can choose to not use it.

Are you ok with that @NicolasAnquetil?