cmd/compile: fix an error in comments

Remove the redundant only.

Change-Id: I9cf2d84ae080a567ad45a2d0ef002c7c89395479
Reviewed-on: https://go-review.googlesource.com/c/go/+/711960
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Bypass: Robert Griesemer <gri@google.com>

Author: mohanson

src/cmd/compile/internal/types2/instantiate.go

@@ -83,7 +83,7 @@ func Instantiate(ctxt *Context, orig Type, targs []Type, validate bool) (Type, e
 //
 // For Named types the resulting instance may be unexpanded.
 //
-// check may be nil (when not type-checking syntax); pos is used only only if check is non-nil.
+// check may be nil (when not type-checking syntax); pos is used only if check is non-nil.
 func (check *Checker) instance(pos syntax.Pos, orig genericType, targs []Type, expanding *Named, ctxt *Context) (res Type) {
 	// The order of the contexts below matters: we always prefer instances in the
 	// expanding instance context in order to preserve reference cycles.