fix: Remove default for minSize in platform buttons

[DFelten]

Since minSize is deprecated it's no longer necessary to add a default for this. The default has to be removed anyway, otherwise the new value minimumSize cannot be set. There is an assert in the CupertinoButton that only allows one of the two values.

The assert:

assert(minimumSize == null || minSize == null)

Without my changes, there is an exception if we try to use minimumSize. It's the same for other buttons.

Example:

PlatformIconButton(
      icon: const Icon(Icons.sort),
      cupertino: (_, _) => CupertinoIconButtonData(minimumSize: Size.square(34)),
    );

[naamapps]

@aqwert Can you please approve and deploy this fix? Thanks