Small refactor of SummaryTab.java to limit scope of warning suppression

As suggested by Stuart here:
https://github.com/openjdk/jdk/pull/6465#discussion_r760694277
if the call to (now deprecated) getObjectPendingFinalizationCount() on L219 is moved to a local variable declaration, the @SuppressWarnings("deprecation") can be moved to apply to just the local variable, instead of the entire formatSummary() method.