remove query_ctx_t detach_client property

remove query_ctx_t detach_client property

Since the removal of NS_QUERY_QCTX_DESTROYED hook, there is no need for
the `qctx->detach_client` object anymore, as this was designed to tell
the plugin whether the client object is about to be, or is already,
freed from memory.  This is not needed anymore, as NS_QUERY_RESET is
called _always_ when the client object is about to be freed from memory.

Remove `detach_client` and tidy up the code a bit by including the
freeing of the qctx object (when allocated) inside the qctx_destroy
function instead of requiring extra calls.