Removed the - signs from default spring var resolution

with the current setup we're using the MDC's default interpolation as a default for spring var resolution (which makes no sense)
with this change we're removing the dashes

fixes gh-1396

spring-cloud-sleuth-samples/spring-cloud-sleuth-sample-feign/src/main/resources/application.yml

@@ -6,7 +6,3 @@ logging.level.com.netflix.discovery: 'OFF'
 sample:
   zipkin:
     enabled: false
-
-spring:
-  application:
-    name: testSleuthFeign