Remove links to gitter.

Closes: #1561

Author: christophstrobl

README.adoc

@@ -1,4 +1,4 @@
-= Spring Data for Apache Cassandra image:https://jenkins.spring.io/buildStatus/icon?job=spring-data-cassandra%2Fmain&subject=Build[link=https://jenkins.spring.io/view/SpringData/job/spring-data-cassandra/] https://gitter.im/spring-projects/spring-data[image:https://badges.gitter.im/spring-projects/spring-data.svg[Gitter]] image:https://img.shields.io/badge/Revved%20up%20by-Develocity-06A0CE?logo=Gradle&labelColor=02303A["Revved up by Develocity", link="https://ge.spring.io/scans?search.rootProjectNames=Spring Data for Apache Cassandra"]
+= Spring Data for Apache Cassandra image:https://jenkins.spring.io/buildStatus/icon?job=spring-data-cassandra%2Fmain&subject=Build[link=https://jenkins.spring.io/view/SpringData/job/spring-data-cassandra/] image:https://img.shields.io/badge/Revved%20up%20by-Develocity-06A0CE?logo=Gradle&labelColor=02303A["Revved up by Develocity", link="https://ge.spring.io/scans?search.rootProjectNames=Spring Data for Apache Cassandra"]
 
 The primary goal of the https://projects.spring.io/spring-data[Spring Data] project is to make it easier to build Spring-powered applications that use new data access technologies such as non-relational databases, map-reduce frameworks, and cloud based data services.
 
@@ -114,7 +114,6 @@ https://docs.spring.io/spring-data/cassandra/docs/current/reference/html/[refere
 * Learn the Spring basics – Spring Data builds on Spring Framework, check the https://spring.io[spring.io] web-site for a wealth of reference documentation.
 If you are just starting out with Spring, try one of the https://spring.io/guides[guides].
 * Ask a question - we monitor https://stackoverflow.com[stackoverflow.com] for questions tagged with https://stackoverflow.com/tags/spring-data[`spring-data-cassandra`].
-You can also chat with the community on https://gitter.im/spring-projects/spring-data[Gitter].
 * Report bugs with Spring Data for Apache Cassandra at https://github.com/spring-projects/spring-data-cassandra/issues[github.com/spring-projects/spring-data-cassandra/issues].
 
 == Reporting Issues