STYLE: Remove deprecated python strel function

I removed the deprecated python strel function
The tests were updated to work directly with FlatStructuringElement
The 4 examples using strel were removed, the 2 binaryfilters were already moved to the ITK examples repo, I will move the 2 grayscalefilter examples soon.

Change-Id: I3fcec5168bcea5697945cc8402e75a1bfeefffea
Merge topic 'review-python-strel'

32ccbe4a STYLE: Remove deprecated python strel function