ts-debian-install: keep avoiding to use pygrub

ts-debian-install: keep avoiding to use pygrub

xen-tools commit 83c37b476a75 ("Start all Debian releases since
Stretch (9) with pygrub by default") started to use pygrub by default.
Revert this.

With "pygrub" setting, xen-create-guest fails on armhf, the
80-install-kernel hook fails because it doesn't know about "armhf".

https://github.com/xen-tools/xen-tools/commit/83c37b476a7534c432ecc9941817aeb989677da6

There's "--nopygrub" but that doesn't work due to several issues, so
removing "pygrub" from "distributions.conf" is the only way.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>