Fix violations of a handful of Error Prone checks (#5519)

Co-authored-by: Daniel Beck <1831569+daniel-beck@users.noreply.github.com>

core/src/main/java/hudson/Functions.java

@@ -236,8 +236,6 @@ public class Functions {
 
     /**
      * Returns a localized string for the specified date, not including time.
-     * @param date
-     * @return
      */
     @Restricted(NoExternalUse.class)
     public static String localDate(Date date) {
@@ -250,9 +248,6 @@ public class Functions {
 
     /**
      * Returns a human-readable string describing the time difference between now and the specified date.
-     *
-     * @param date
-     * @return
      */
     @Restricted(NoExternalUse.class)
     public static String getTimeSpanString(Date date) {