Remove NoFinalize from the checkstyle rule

- We sometimes use it, although it's very rare.

src/main/resources/io/netty/checkstyle.xml

@@ -105,7 +105,6 @@
     -->
     <module name="SimplifyBooleanExpression"/>
     <module name="SimplifyBooleanReturn"/>
-    <module name="NoFinalizer"/>
     <module name="SuperFinalize"/>
     <module name="PackageDeclaration"/>
     <module name="ExplicitInitialization"/>