Remove registration side-effect test case

Keep the unused-variable tests focused on maybe_unused cases.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jketema

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticVariables/test.cpp

@@ -14,16 +14,6 @@ extern const int constVar9 = 9; // GOOD
 static int staticVar10 = 10; // $ SPURIOUS: Alert // GOOD [FALSE POSITIVE] (referenced in a never instantiated template)
 [[maybe_unused]] static int staticVar11; // GOOD (explicitly marked as unused)
 
-static int registrationCount;
-
-int* registerBenchmark()
-{
-	++registrationCount;
-	return &registrationCount;
-}
-
-static int* benchmarkRegistration = registerBenchmark(); // GOOD (initializer has a side effect)
-
 void f()
 {
 	int *ptr = &staticVar4;