scipyoptdoc.py: Remove unused variable 'sixu'

The [unused __sixu__](https://github.com/scipy/scipy/search?q=sixu&unscoped_q=sixu) is no longer required because Scipy no longer supports Python2 and __unicode()__ is removed in Python 3.

doc/source/scipyoptdoc.py

@@ -39,12 +39,6 @@
 from scipy._lib._util import getargspec_no_self
 
 
-if sys.version_info[0] >= 3:
-    sixu = lambda s: s
-else:
-    sixu = lambda s: unicode(s, 'unicode_escape')
-
-
 def setup(app):
     app.add_domain(ScipyOptimizeInterfaceDomain)
     return {'parallel_read_safe': True}