UCT/MM: Fix endpoint flush - don't update cached tail

We must not update cached tail in uct_mm_ep_flush(), because we don't
execute the pending queue. As a result, we may get new send resources
but not use them, so flush could return UCS_OK while there are still
pending requests.

Fixes openucx#3052

src/uct/sm/mm/mm_ep.c

@@ -417,8 +417,6 @@ ucs_status_t uct_mm_ep_flush(uct_ep_h tl_ep, unsigned flags,
 {
     uct_mm_ep_t *ep = ucs_derived_of(tl_ep, uct_mm_ep_t);
 
-    uct_mm_ep_update_cached_tail(ep);
-
     if (!uct_mm_ep_has_tx_resources(ep)) {
         return UCS_ERR_NO_RESOURCE;
     }