Make Simplification API more conservative

Currently many lower level simplification methods are visible.

While I was porting CALCITE-2247 to a branch near 1.16 I've bumped into an issue because of the fact that by calling a specific simplify method may leave out some otherwise applicable simplifications.

For master there is already an extra safety feature by the presence of CALCITE-2205; it seems like using less entry points may even lead to better simplifications - by changing this; a filter have been removed.

https://github.com/kgyrtkirk/calcite/commit/2e29a659792f6bd9419dc0f97bf5a3bdd9f6f2cc