Remove JDK 1.7 guards
Improve SubscriptionRegistry subscription removal

This a backport of:
https://github.com/spring-projects/spring-framework/commit/ce20abde51ac52dde2f318e28004af80cca886fe
https://github.com/spring-projects/spring-framework/commit/083d415fc4d54ce19f7939b3d837f1f1eda5732a

Issue: SPR-11931, SPR-11930