Spring Cleaning in December

- Delete unused imports
- Delete unused code
- Clean up warnings

spring-context/src/main/java/org/springframework/format/datetime/standard/PeriodFormatter.java

@@ -17,7 +17,6 @@
 package org.springframework.format.datetime.standard;
 
 import java.text.ParseException;
-import java.time.Instant;
 import java.time.Period;
 import java.util.Locale;