vtkQWidgetTexture: remove unused QOpenGL include
also resolves this compiler warning
#warning: To use GLEW with Qt, do not include <qopengl.h> or <QOpenGLFunctions> after glew.h [-W#warnings]
#warning To use GLEW with Qt, do not include <qopengl.h> or
<QOpenGLFunctions> after glew.h