build: cleanup possibly dangling Python 3 host symlink

build: cleanup possibly dangling Python 3 host symlink

When switching from master branch to 19.07 or older, we need to ensure
that Python symlink in staging bin directory points to Python 2.

We can't rely completly just on SetupHostCommand as its executed only in
cases when the $(STAGING_DIR_HOST)/bin/python doesn't already exist, so
we need to remove it before running SetupHostCommand.

This is a cherry-pick of 3b68fb57c938af3948ae4c2da61501183fbef649
with python3 instead of python2

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>