cmd/compile/internal/types: remove TTYPEPARAM and TUNION types

cmd/compile/internal/types: remove TTYPEPARAM and TUNION types

These were used by the nounified frontend for representing
uninstantiated generic types; however, the unified frontend only needs
types1 to represent instantiated types.

Updates #57410.

Change-Id: Iac417fbf2b86f4e08bd7fdd26ae8ed17395ce833
Reviewed-on: https://go-review.googlesource.com/c/go/+/458621
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>