When sub-planning, we pass a caching callback to decide if we want to cache per-branch and/or whole query planning results.

A common callback object that is used is ClassicPlanCacheWriter.

This class has a "executeInSbe" flag that is set to false in classic planners and set to true in "classic planners for sbe". 

However, the only thing that this flag affects is what metric are we writing: reads for SBE or works for Classic.

This code can be simplified and flag can be removed if we just write both numbers.

This is also important if we want to move engine selecting after planning: this will allow us to trial a cached plan no matter what engine we choose.

Also it will allow us not to flush plan cache on internalQueryFrameworkControl parameter change.