Removes api.ImportRenamer for a different approach to "env" (#680)

Before, we introduced a type `api.ImportRenamer` to resolve conflicts
where the "env" module was shared between AssemblyScript and
user-defined functions. This API was never used in GitHub, and is
complicated.

AssemblyScript also isn't the only ABI to share the "env" module, as
other web APIs like Emscripten do also. The less complicated approach is
to have packages that need to share "env" use
`ModuleBuilder.ExportFunctions` instead, and use namespaces as needed if
there is overlap.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

runtime.go

@@ -174,13 +174,6 @@ func (r *runtime) CompileModule(ctx context.Context, binary []byte, cConfig Comp
 		return nil, err
 	}
 
-	// Replace imports if any configuration exists to do so.
-	if importRenamer := config.importRenamer; importRenamer != nil {
-		for _, i := range internal.ImportSection {
-			i.Module, i.Name = importRenamer(i.Type, i.Module, i.Name)
-		}
-	}
-
 	internal.AssignModuleID(binary)
 
 	if err = r.store.Engine.CompileModule(ctx, internal); err != nil {