scripts/buildhistory-diff: drop use of distutils

The use of distutils.version.LooseVersion to check for GitPython > 0.3.1
is not really needed anymore since any supported distribution has at least
1.0.0 (centos-7 via epel7, debian-9, ubuntu-16.04)

If we want to reinstate this check, alternatives would be to require
python3-packaging on all hosts and use packaging.version.Version or
use an imported LooseVersion in bb.version.

[YOCTO #14610]

Signed-off-by: Tim Orling <timothy.t.orling@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit bc90dcae)
Signed-off-by: Steve Sakoman <steve@sakoman.com>