structQualifierErrorCheck is called incorrectly in glslang.y. Remove one
unnecessary call entirely and change another call to samplerErrorCheck.
This will enable further changes to merge structQualifierErrorCheck into
singleDeclarationErrorCheck.

TEST=angle_unittests
BUG=angleproject:969

Change-Id: Ib43233e275fbf4db7e5fda6df6b45a655bdee8a2
Reviewed-on: https://chromium-review.googlesource.com/265200Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>