Remove irr namespace (#16309)

irr/include/quaternion.h

@@ -19,8 +19,6 @@
 // - For uses of getMatrix() you have to use quaternion::getMatrix_transposed instead.
 // #define IRR_TEST_BROKEN_QUATERNION_USE
 
-namespace irr
-{
 namespace core
 {
 
@@ -704,4 +702,3 @@ inline core::quaternion &quaternion::rotationFromTo(const vector3df &from, const
 }
 
 } // end namespace core
-} // end namespace irr