Delete outdated paragraph about OptimizedUnicode again

sqlite3.OptimizedUnicode is an alias for str in Python 3 and
its behavior is already tested in CheckOptimizedUnicode in
Lib/sqlite3/test/factory.py.

sqlite3.OptimizedUnicode was undocumented in 0fc10a33eb4c and
probably added back with the result of a bad merge.