Revert "fsharplint.json: temporarily enable new rule for SelfCheck"

This reverts commit fa8c654,
because we need to make the setting be in sync with the documentation
before we publish a new release.

(We had enabled the rule temporarily so that when rebasing PR#637, CI
would flag it and the author would address it :) )

Author: knocte

src/FSharpLint.Core/fsharplint.json

@@ -286,7 +286,7 @@
     "uselessBinding": { "enabled": true },
     "tupleOfWildcards": { "enabled": true },
     "favourTypedIgnore": { "enabled": false },
-    "favourNonMutablePropertyInitialization": { "enabled": true },
+    "favourNonMutablePropertyInitialization": { "enabled": false },
     "favourReRaise": { "enabled": true },
     "favourStaticEmptyFields": { "enabled": false },
     "favourConsistentThis": {