fix: remove requirement for static closures

`SlevomatCodingStandard.Functions.StaticClosure` does not provide a setting to
toggle this sniff for only arrow functions or traditional `function` closure
syntax.

While declaring traditional closures static *might* be a best practice, the PHP
RFC introducing the arrow function implementation implies that declaring
`static` arrow functions is a micro-optimization in the majority of cases:

> For normal closures, [`$this` binding] can be prevented by prefixing them with
`static`. For the sake of completeness this is also supported for arrow
functions...

and:

> Static closures are rarely used: They're mainly used to prevent `$this`
cycles, which make GC behavior less predictable. Most code need not concern
itself with this.

<https://wiki.php.net/rfc/arrow_functions_v2#this_binding_and_static_arrow_functions>

I would prefer to avoid the addition of `static` for arrow functions, as they
are, by design, intended to provide a concise syntax *without* `static`.

Kleinweb/ruleset.xml

@@ -201,7 +201,6 @@ them to our own mu-plugin or standard library.
     </rule>
     <rule ref="SlevomatCodingStandard.Functions.DisallowEmptyFunction"/>
     <rule ref="SlevomatCodingStandard.Functions.RequireArrowFunction"/>
-    <rule ref="SlevomatCodingStandard.Functions.StaticClosure"/>
     <rule ref="SlevomatCodingStandard.Functions.UnusedInheritedVariablePassedToClosure"/>
     <rule ref="SlevomatCodingStandard.Functions.UnusedParameter"/>
     <rule ref="SlevomatCodingStandard.Functions.UselessParameterDefaultValue"/>