Revert "Add some defensive coding to av_store()"

Revert "Add some defensive coding to av_store()"

This reverts commit bc62bf8519f9005df2fb29dbd3d330202b258b6b.

As Dave Mitchell said in <Perl/perl5/issues/17265/570253376@github.com>

"The docs for av_store() say that the caller is responsible for first
incrementing the ref count of the new SV before passing it to
av_store(). In the normal case of the two elements being different,
av_store() will decrement the old SV before storing the new one, and
everything is good. When the two elements are the same SV, av_store()
*still* needs to decrement the ref count, to undo the increment the
caller just did."

This should resolve GH #17265