Refactoring only: remove [Id] field from ForeignStubs

We used to pass the list of top-level foreign exported bindings to the
code generator so that it could create StablePtrs for them in the
stginit code.  Now we don't use stginit unless profiling, and the
StablePtrs are generated by C functions marked with
attribute((constructor)).  This patch removes various bits associated
with the old way of doing things, which were previously left in place
in case we wanted to switch back, I presume.  Also I refactored
dsForeigns to clean it up a bit.