k_mem_pool_alloc(): remove dead code

A loop in k_mem_pool_alloc() around z_sys_mem_pool_block_alloc() assumes
the later may return -EAGAIN with an elaborate comment about it. But
-EAGAIN is no longer returned by that function since commit 7845e1b0
("lib/mempool: Fix spurious -ENOMEM due to agressive latency control").

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>