patch 9.0.2039: completion shows current word after completion restart

patch 9.0.2039: completion shows current word after completion restart

Problem:  completion shows current word after completion restart
Solution: remove the word being completed after completion restart

The word being completed is shown in a completion list after completion
restart, because it isn't removed from the current buffer before
searching for matches.

Also adjust `Test_complete_add_onechar` to match the new behavior.

closes: #13349

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: nwounkn <nwounkn@gmail.com>