Linker does not remove unused fields initialisers. There are quite few examples with code which is never used by simple app (e.g hello world) but still not removed by linker in mscorlib.
With the exception of the bindings, which are marked [CompilerGenerated] for safety, the linker does not re-write IL because it can be pretty complex.
What happens is that the fields are not really unused, from the linker perspective, and get marked because:
a) the type, e.g. System.GC, was marked;
b) the static constructor is marked (because the type was marked and the static ctor, if present, needs to be run);
c) the IL scan of the .cctor marks every field being used (the stsfld);
In this case (GC) it's a good thing since the field is initialized to the return value of an internal call - and the linker cannot know if that call has side effects (even if the field is never used elsewhere).
There are other cases where the fields could have been:
* assigned values that would not have side effects (that removes calls, including object creation); and
* are not used outside the .cctor
To deal with them we would need to change how marking is done - not impossible but not easy either. If there's not many of them we could have special rules for the most common/costly .cctors.
This bug was targeted for a past milestone, moving to the next non-hotfix active milestone.