Do not drop voip_vscs if it doesn't exist

This fixes broken 9458.up script:

| ERROR 1051 (42S02) at line 3: Unknown table `voip_vscs'

From: Michael Prokop <mprokop@sipwise.com>

db_scripts/diff/9458.up

@@ -1,6 +1,7 @@
 USE provisioning;
 
-DROP TABLE voip_vscs;
+-- voip_vscs was probably already dropped with 9069.up
+DROP TABLE IF EXISTS voip_vscs;
 
 -- voip_audio_files was probably already dropped with 9069.up
 DROP TABLE IF EXISTS voip_audio_files;