FIX: LGTM warning: Unnecessary 'else' clause in loop

This 'for' statement has a redundant 'else' as no 'break' is present in the body.

https://lgtm.com/rules/5980098/

Author: DimitriPapadopoulos

nibabel/cifti2/cifti2.py

@@ -1156,8 +1156,7 @@ def get_index_map(self, index):
             a2md = self._get_indices_from_mim(v)
             if index in a2md:
                 return v
-        else:
-            raise Cifti2HeaderError("Index not mapped")
+        raise Cifti2HeaderError("Index not mapped")
 
     def _validate_new_mim(self, value):
         if value.applies_to_matrix_dimension is None: