box: fix memleak on functional index drop

We just don't free functional index keys on functional index drop now.
Let's approach keys deletion as in the case of primary index drop ie
let's drop these keys in background.

We should set `use_hint` to `true` in case of MEMTX_TREE_VTAB_DISABLED
tree index methods because `memtx_tree_disabled_index_vtab` uses
`memtx_tree_index_destroy<true>`. Otherwise we get read outside of index
structure for stub functional index on destroy for introduced `is_func`
field (which is reported by ASAN).

Closes #10163

NO_DOC=bugfix