Remove internal self-replicating support from Tasks

Remove internal self-replicating support from Tasks

In the initial TPL prototype, Tasks had a feature that allowed them to "replicate", meaning to schedule a copy of themselves.  Prior to shipping the initial version, this feature was made internal, as it was deemed unnecessary, but by then the Parallel loops constructs had already been built on top of them.  Now that Parallel no longer depends on this feature (and isn't in CoreLib), we can entirely rip it out.

Commit migrated from https://github.com/dotnet/coreclr/commit/d433a7b4443c7a6eee389c6cb57a21c7e307630a