go/parser: remove import path string syntax checking

go/parser: remove import path string syntax checking

The validity of an import path string is checked by the type checker
(and possibly other tools); it doesn't need to be done by the parser.
Remove the respective code and tests.

Also, adjust a corresponding go/types test which resolves a TODO.

For #54511.

Change-Id: Id1fc80df4e3e83be3ef123da3946ccb8f759779f
Reviewed-on: https://go-review.googlesource.com/c/go/+/424855
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>