CustomBuildSystemConfigWidget: clear configs before reloading.

`loadFrom()` is called on an already-loaded widget when resetting,
 for example after choosing "Discard" when changing to another
 config page.
The existing configs must be cleared before loading new ones.

In debug mode, this caused a crash due to a later assertion failure:
BUG: 376523

Otherwise, duplicate configs seem likely to cause some glitches.