mergify: remove duplicated config and use the default ones for the backport labels

See https://github.com/elastic/logstash/pull/17740#discussion_r2167764698

.mergify.yml

@@ -68,13 +68,8 @@ pull_request_rules:
       - label=backport-8.16
     actions:
       backport:
-        assignees:
-          - "{{ author }}"
         branches:
           - "8.16"
-        labels:
-          - "backport"
-        title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})"
 
   - name: backport patches to 8.17 branch
     conditions:
@@ -83,13 +78,8 @@ pull_request_rules:
       - label=backport-8.17
     actions:
       backport:
-        assignees:
-          - "{{ author }}"
         branches:
           - "8.17"
-        labels:
-          - "backport"
-        title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})"
 
   - name: backport patches to 8.18 branch
     conditions:
@@ -98,13 +88,8 @@ pull_request_rules:
       - label=backport-8.18
     actions:
       backport:
-        assignees:
-          - "{{ author }}"
         branches:
           - "8.18"
-        labels:
-          - "backport"
-        title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})"
 
   - name: backport patches to 8.19 branch
     conditions:
@@ -123,10 +108,5 @@ pull_request_rules:
       - label=backport-9.0
     actions:
       backport:
-        assignees:
-          - "{{ author }}"
         branches:
           - "9.0"
-        labels:
-          - "backport"
-        title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})"