mgcp: remove unused callback pointer

struct mgcp_config contains a function pointer realloc_cb, which is
never popoulated nor used anywhere in the code. Lets remove it

Change-Id: I8d58281e1ff898638293c9e8cb000329462c7a70
diff --git a/TODO-RELEASE b/TODO-RELEASE
index c5a3b36..741ab14 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -24,3 +24,4 @@
 # If any interfaces have been removed or changed since the last public release, a=0.
 #
 #library		what		description / commit summary line
+libsomo-mgcp		ABI change	mgcp.h, struct mgcp_config, remove realloc_cb (unused)
\ No newline at end of file