Conditional directives and ActivitityDisposable are no longer needed

[T-Gro]

This related to chores around nullness, which needed to branch while "last known good" compiler for initial bootstrapping had bugs around 'use' and nullable disposable, especially used in Activity.fs handling.

This full section in NullnessShims.fs can be removed

#if BUILDING_WITH_LKG
    type ActivityDisposable = System.IDisposable
#else
    type ActivityDisposable = System.IDisposable | null
#endif

and all code usages of ActivityDisposable can be directly replaced with System.IDisposable | null on their place of usages.
This should especially mean Activity.fs and Activity.fsi , but there might be others.