Autogen: Revert adding ui files to byproducts of the timestamp target

When `ui_` include directives have different paths than `CMakeLists.txt`,
the dependency graph is not generated correctly.  This is the root cause
of #16776.  However, when #26135 was fixed by commit 5363bebc1e (Autogen:
Fix compilation of unchanged source files, 2024-07-16, v3.31.0-rc1~328^2),
it made #16776 worse: the build is always dirty.  Revert the fix for now.

Fixes: #26485
Issue: #26135
Issue: #25436