Revert "[prdoc] Require SemVer bump level" (#3987)

Reverts paritytech/polkadot-sdk#3816

prdoc/schema_user.json

@@ -132,8 +132,7 @@
           }
         },
         "required": [
-          "name",
-          "bump"
+          "name"
         ]
       },
       "migration_db": {
@@ -188,11 +187,6 @@
             "const": "patch",
             "title": "Patch",
             "description": "A bump to the third leftmost non-zero digit of the version number."
-          },
-          {
-            "const": "none",
-            "title": "None",
-            "description": "This change requires no SemVer bump (e.g. change was a test or benchmark)."
           }
         ]
       },