Fix crash in animation export in compositions docker with empty config

Remove unused imageExtension, which calculation was crashing when ANIMATION_EXPORT settings didn't exist.

Test Plan

• Remove ExportConfiguration-ANIMATION_EXPORT line from kritarc config file and restart krita
• Export animation from compositions docker

Krita exported animation without crashing

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.