go/types: delete unnecessary slice construction

CL 374294 made our check for incorrect type parameters constraints
eager, but failed to remove the construction of the bounds slice, which
was no longer used.

Change-Id: Ib8778fba947ef8a8414803e95d72c49b8f75c204
Reviewed-on: https://go-review.googlesource.com/c/go/+/386717


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>