Clean up the rules for making vbmeta.img.

This CL intends to be a no-op change to functionality. It prepares for
the upcoming changes that support creating chained VBMeta images.

Bug: 112007947
Test: `m dist` with aosp_taimen-userdebug
Change-Id: Iaac45bde7971b056e17a6e7b0c89336ed206916e