Remove `g:textobj_function_django_template`

Drop the (probably) unused function `textobj_function_django_template`
and use a script-local function for avoiding to redefine the functions.

Fixes https://github.com/mjbrownie/django-template-textobjects/issues/3
Merge pull request #5 from blueyed/fix-E128

Remove `g:textobj_function_django_template`