[pylint] fix global-variable-not-assigned issues

If there is no write access, there is no need for global.  Remove global
statement if there is no assignment.

global-variable-not-assigned:
  Using global for names but no assignment is done Used when a variable is
  defined through the "global" statement but no assignment to this variable is
  done.

In Pylint 2.11 the global-variable-not-assigned checker now catches global
variables that are never reassigned in a local scope and catches (reassigned)
functions [1][2]

[1] https://pylint.pycqa.org/en/latest/whatsnew/2.11.html
[2] https://github.com/PyCQA/pylint/issues/1375



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>