How can I ask a journal to use my figures, knowing that the journal is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License?
If it is unnecessary, how can I do it properly?
Is it necessary? No
You can contact the editor. However, in the scenario you painted, that'll be mere formalities.
Your article was published under the CC-SA, the licensor already grants the licensee the right to use and modify.
You are free to:
The licensor cannot revoke these freedoms as long as you follow the license terms. Under the following terms:
The general answer to the headline question is yes you need permission from the copyright holder even for your own work if you have given up copyright. But, in almost all cases the original creator will be given a license for certain uses that usually includes such things as reuse in a dissertation. Note, also, that copyright law differs around the world as do the policies of publishers. Again, in the general case, the author should check the license they were given when they signed over copyright.
However, in your case, you already have permission from the CC license you mention (CC-BY-SA), so you don't need to enquire further. But that license remains for your reuse and it should be so stated in your citation.
And, yes, you need to cite the earlier use along with the license.