qt: Remove build date in about dialog in gitian builds #3500

In About... it shows the build date.

In deterministic builds this either shows nonsense, or makes deterministic building impossible (at least over multiple days).

Should be removed or replaced with date of last commit (if possible)...

@wtogami? Huh...

I looked at the "About..." window in a gitian build and did not notice a build date.

Closing for now.