basic/strv: drop strv_free_free

basic/strv: drop strv_free_free

I think the function name is confusing: we generally say "free_free" when
both keys and values are freed in a hash map, but here the type is an
array of strvs, so the name should be something like strv_array_free.

The function is unused since 143fadf369a18449464956206226761e49be1928 (2018),
let's just drop it.