[JENKINS-62669] Removing Mercurial support (#320)

* [JENKINS-62669] Removing Mercurial support

* Javadoc fix

* Some more deprecations

* Help text adjustment

Author: jglick

pom.xml

@@ -111,11 +111,6 @@
       <groupId>org.jenkins-ci.plugins</groupId>
       <artifactId>git</artifactId>
     </dependency>
-    <dependency>
-      <groupId>org.jenkins-ci.plugins</groupId>
-      <artifactId>mercurial</artifactId>
-      <version>2.8</version>
-    </dependency>
     <dependency>
       <groupId>org.jenkins-ci.plugins</groupId>
       <artifactId>jackson2-api</artifactId>