[m-dev.] diff: fix in polymorphism.m

[Simon Taylor]

> Estimated hours taken: 0.1
> 
> compiler/polymorphism.m:
> 	Remove a very suspicious looking application of a *variable* type
> 	mapping to a type with *type variables* in it. The code doesn't seem
> 	to have any purpose, and all the test cases still pass.
 
Thanks DJ.