removed explicit DeprecationWarning for AtomGroup["name"]

It turns out that with this warning in place we raise two deprecation warnings because we also
have a warning on _get_named_atom(item).

Example output:

 >>> w.atoms['AAA']
 .../mdanalysis/package/MDAnalysis/core/groups.py:1520: DeprecationWarning: Using the [] operator with strings is deprecated and will be removed in 1.0. Please use `select_atoms('name AAA')` instead.
   category=DeprecationWarning)
 .../mdanalysis/package/MDAnalysis/core/groups.py:1522: DeprecationWarning: `_get_named_atom` is deprecated!
 Instant selector AtomGroup['<name>'] or AtomGroup.<name> is deprecated and will be removed in 1.0. Use AtomGroup.select_atoms('name <name>') instead.
   return self._get_named_atom(item)
 <Atom 1: AAA of resname RsA, resid 1 and segid SegA>

package/MDAnalysis/core/groups.py

@@ -1513,11 +1513,6 @@ def __getitem__(self, item):
         #
         # u.atoms['HT1'] access, otherwise default
         if isinstance(item, string_types):
-            warnings.warn("Using the [] operator with strings is deprecated "
-                          "and will be removed in 1.0. "
-                          "Please use `select_atoms('name {}')` "
-                          "instead.".format(item),
-                          category=DeprecationWarning)
             try:
                 return self._get_named_atom(item)
             except (AttributeError, selection.SelectionError):