Remove mi_hpc field from interface files

The `mi_hpc` field is not used for anything as far as I can discern so
there is no reason to record in the private interface of a module that
there are modules in the transitive closure which use `hpc`.

You can freely mix modules which use `-fhpc` and ones which don't.

Whether to recompile a module due to `-fhpc` being passed to the module
itself is determined in `fingerprintDynFlags`.