Remove outdated warning about OpenMP < 3.0. Even the now-EOL Docker images for...

Remove outdated warning about OpenMP < 3.0. Even the now-EOL Docker images for manylinux2010 had that.

Closes https://github.com/cython/cython/issues/4574

Cython/Compiler/Nodes.py

@@ -8989,10 +8989,6 @@ class ParallelRangeNode(ParallelStatNode):
             self.index_type = PyrexTypes.c_py_ssize_t_type
         else:
             self.index_type = self.target.type
-            if not self.index_type.signed:
-                warning(self.target.pos,
-                        "Unsigned index type not allowed before OpenMP 3.0",
-                        level=2)
 
         # Setup start, stop and step, allocating temps if needed
         self.names = 'start', 'stop', 'step'