go/parser: clean up unnecessary arguments and replace an if statement

Eliminate an unnecessary argument from parseGenericType, and replace an
if statement with a switch.

Change-Id: Iaa8afeface929332579f183c8e523961cca9aca4
Reviewed-on: https://go-review.googlesource.com/c/go/+/354869


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