Revert "MT#9671 Add search of subscribers by customer external_id too."

This reverts commit a2b4c627f3.

Reverted to check the cause of the failed api tests.
Will be returned just after build if is not a reason for the failed tests.

lib/NGCP/Panel/Controller/API/Subscribers.pm

@@ -83,19 +83,6 @@ class_has 'query_params' => (
                 },
             },
         },
-        {
-            param => 'customer_external_id',
-            description => 'Filter for subscribers of a specific customer external_id.',
-            query => {
-                first => sub {
-                    my $q = shift;
-                    return { 'contract.external_id' => { like => $q } };
-                },
-                second => sub {
-                    return { join => 'contract' };
-                },
-            },
-        },
         {
             param => 'is_pbx_group',
             description => 'Filter for subscribers who are (not) pbx_groups.',