doc: Update a PTL guide
doc: Update a PTL guide

It does not need anymore to add database migration placeholders
because of using alembic.
So remove the description in the PTL guide.

Change-Id: If958dd78ff82e2239be1af3835a51a1a3551c5d9
Signed-off-by: Takashi Natsume <2c48eb5bf73c355a2d5583bc776dffa383cdf819@gmail.com>

openstack/nova,openstack/nova,mahak/nova,mahak/nova,openstack/nova,mahak/nova