export: remove "error_unpack_unsafe" from "exports"

"error_unpack_unsafe" was removed from export in commit [1]
and accidentally reanimated during rebase [2].
Let's remove "error_unpack_unsafe" from "exports".

1. https://github.com/tarantool/tarantool/commit/6aafa697e1ec8166df721573195711cea5ec3135
2. https://github.com/tarantool/tarantool/commit/5ceabb378d0169dc776449e45577515114e39f12

Follow-up #5932