Revert "Set bot_account settings in .mergify.yml (#2395)"

This reverts commit e4964df.

Setting it to `clash-lang-builder` results in (unresolvable) errors on
PRs. Setting it to the recommended `{ author }` yields a "not a valid
value for dictionary value ..." error. I guess we'll have to live with
Mergify's warnings for now.

.mergify.yml

@@ -9,7 +9,6 @@ pull_request_rules:
       - merged
     actions:
       copy:
-        bot_account: clash-lang-builder
         ignore_conflicts: True
         branches:
           - "1.2"
@@ -21,7 +20,6 @@ pull_request_rules:
       - merged
     actions:
       copy:
-        bot_account: clash-lang-builder
         ignore_conflicts: True
         branches:
           - "1.4"
@@ -33,11 +31,8 @@ pull_request_rules:
       - merged
     actions:
       copy:
-        bot_account: clash-lang-builder
         ignore_conflicts: True
         branches:
           - "1.6"
       queue:
-        update_bot_account: clash-lang-builder
-        merge_bot_account: clash-lang-builder
         update_method: rebase