msc_mgw_setup: use mgcp_client_pool_empty()

Don't fall back to the legacy config if the pool is configured but no
connection to any pool member can be established.

Depends: osmo-mgw I009483ac9dfd6627e414f14d43b89f40ea4644db
Related: OS#5993
Change-Id: I44e7b2723d801ceb03aaa2e5546802b4eb56b3c3
diff --git a/TODO-RELEASE b/TODO-RELEASE
index d0852fc..a2695f2 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -7,3 +7,4 @@
 # If any interfaces have been added since the last public release: c:r:a + 1.
 # If any interfaces have been removed or changed since the last public release: c:r:0.
 #library	what		description / commit summary line
+libosmo-mgcp-client > 1.11.0	mgcp_client_pool_empty()