go/parser: leave checking of LHS in short var decls to type checker

go/parser: leave checking of LHS in short var decls to type checker

Instead of checking at parse-time that the LHS of a short variable
declaration contains only identifiers, leave the check to the the
type checker which tests this already.

This removes a duplicate error and matches the behavior of the
syntax package.

For #54511.

Change-Id: I4c68f2bd8a0e015133685f9308beb98e714a83fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/426476
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>