python: remove broken/ineffectual keywords marks initialization
python: remove broken/ineffectual keywords marks initialization

By my analysis, this deleted code block has no effect:

1. `self.keywords` is `update`d with `callspec.marks`.
2. `self.own_markers` is `update`d with `callspec.marks`.
3. `self.keywords` is `update`d with `self.own_markers`.

So together steps 2+3 completely undo step 1.

RonnyPfannschmidt/pytest,pytest-dev/pytest,markshao/pytest,Akasurde/pytest