Don't provide twice MySQL image

Summary:
Fixes the following issue:

found conflicting ID 'nasqueron/mysql'
  in "<unicode string>", line 40, column 1:
    nasqueron/mysql:
    ^

Test Plan: salt equatower state.apply roles/paas-docker/docker/images

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1767