cmd-export: Remove reference to MySQL and PostgreSQL
cmd-export: Remove reference to MySQL and PostgreSQL

ReviewDb is gone, hence MySQL and PostgreSQL are no longer used.

Change-Id: I2fa3ad412522c63ee71e2f8df073afc13fabe463
Signed-off-by: Edwin Kempin <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>

GerritCodeReview/plugins_reviewnotes