Simplify MediaWiki nginx configuration for thumbnails

Summary:
The current configuration contains a if block.

Simplified by a try_files directive.

https://shorturls.redwerks.org/ concurs, it also switched the previous
configuration to try_files.

Test Plan:
Tested for:

• www.wolfplex.org (wiki URL scheme)
• agora.ansqueron.org (root URL scheme)

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1489