Remove <tt></tt> usage from Class and ClassLoader

The javadoc for java.lang.Class and java.lang.ClassLoader has some usages of the <tt></tt> HTML tag. This tag is not part of HTML 5 and has been replaced with <code></code>. In javadoc, direct use of <tt></tt> or <code></code> is rarely necessary since the {@code } tag can be used instead. In addition, ClassLoader has multiple extraneous usage of <tt></tt> inside the target of a {@link} tag; these should be removed.