Shaders: don't assert for ARB_uniform_buffer_object presence twice.

Done this way only in the Phong shader, everywhere else it's just the
MAGNUM_ASSERT_GL_EXTENSION_SUPPORTED() macro. Some WIP code that I
forgot to clean up?

src/Magnum/Shaders/PhongGL.cpp

@@ -156,11 +156,6 @@ PhongGL::CompileState PhongGL::compile(const Configuration& configuration) {
 
     const GL::Context& context = GL::Context::current();
 
-    #ifndef MAGNUM_TARGET_GLES
-    CORRADE_ASSERT(!(configuration.flags() >= Flag::UniformBuffers) || context.isExtensionSupported<GL::Extensions::ARB::uniform_buffer_object>(),
-        "Shaders::PhongGL: uniform buffers require" << GL::Extensions::ARB::uniform_buffer_object::string(), CompileState{NoCreate});
-    #endif
-
     #ifndef MAGNUM_TARGET_GLES
     const GL::Version version = context.supportedVersion({GL::Version::GL320, GL::Version::GL310, GL::Version::GL300, GL::Version::GL210});
     #else