Use Threadlocal.remove instead of .set(null)

See https://github.com/spring-cloud/spring-cloud-sleuth/issues/27 for
reference.

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -1386,7 +1386,7 @@ public class SpringApplication {
 			return action.get();
 		}
 		finally {
-			applicationHook.set(null);
+			applicationHook.remove();
 		}
 	}
 

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/convert/CharSequenceToObjectConverter.java

@@ -68,7 +68,7 @@ class CharSequenceToObjectConverter implements ConditionalGenericConverter {
 			return this.conversionService.canConvert(STRING, targetType);
 		}
 		finally {
-			this.disable.set(null);
+			this.disable.remove();
 		}
 	}