GLSL validation: don't print error message twice.

Without this change, all GLSL validation failures look like this:

Error parsing GLSL shader:
error: Error parsing GLSL shader:

It doesn't seem possible to get rid of the "error:" at the start of
the message (which would have made this a code-only change), but it is
possible to get rid of the double message.

Update GLSL tint test results.

Change-Id: I8679b7b4d98e3779c8abf0c056afb9ec572eb870
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/156280
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>