kwinglutils: Drop GLRenderTarget::setTextureDirty()

It's effectively unused and removing it allows us to get rid of
GLTexture field, which is very useful for abstracting the concept of a
"render target" across OpenGL and QPainter backends.

src/libkwineffects/kwinglutils.cpp

@@ -964,8 +964,6 @@ void GLRenderTarget::pushRenderTargets(QStack <GLRenderTarget*> targets)
 GLRenderTarget* GLRenderTarget::popRenderTarget()
 {
     GLRenderTarget* ret = s_renderTargets.pop();
-    ret->setTextureDirty();
-
     if (!s_renderTargets.isEmpty()) {
         s_renderTargets.top()->bind();
     }

src/libkwineffects/kwinglutils.h

@@ -425,10 +425,6 @@ public:
         return mValid;
     }
 
-    void setTextureDirty() {
-        mTexture.setDirty();
-    }
-
     static void initStatic();
     static bool supported()  {
         return sSupported;