Delete .upb file after rendering to clean up

[seananderson]

Describe the bug

A .upb file gets left behind when rendering. It would be nice to clean this up for the user.

To Reproduce

Build a resdoc or tech report. E.g. from the unit tests.

Expected behavior

No extra files get left behind.

Observed behavior

A file, such as techreport.upb gets left behind.

Screenshots