all: drop emacs file variables from source files

We no longer add these. If you use Emacs, configure it yourself.

Also, due to our "smart-tab" usage the editor anyway does a subpar
job handling our tabs. However, on the upside every user can choose
whatever tab-width he/she prefers. If "smart-tabs" are used properly
(like we do), every tab-width will work.

No manual changes, just ran commands:

    F=($(git grep -l -e '-\*-'))
    sed '1 { /\/\* *-\*-  *[mM]ode.*\*\/$/d }'     -i "${F[@]}"
    sed '1,4 { /^\(#\|--\|dnl\) *-\*- [mM]ode/d }' -i "${F[@]}"

Check remaining lines with:

    git grep -e '-\*-'

The ultimate purpose of this is to cleanup our files and eventually use
SPDX license identifiers. For that, first get rid of the boilerplate lines.

examples/python/gi/add_connection.py

@@ -1,5 +1,4 @@
 #!/usr/bin/env python
-# -*- Mode: Python; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 # vim: ft=python ts=4 sts=4 sw=4 et ai
 
 #