avocado.core.tree: Remove useless code

path_parent returns parent, the extra return won't affect the result.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/tree.py

@@ -512,8 +512,6 @@ def path_parent(path):
     :return: the parent path as string.
     """
     parent = path.rpartition('/')[0]
-    if parent == '':
-        return ''
     return parent