AXIS2-6106 Remove unmaintained XMPP transport module

The XMPP transport depends on jivesoftware:smack (circa 2008) which
has not been updated in 18 years. The modern Smack library
(org.igniterealtime.smack 4.x) has a completely incompatible API.
No known users of Axis2 XMPP transport exist.

Removed:
- modules/transport/xmpp (14 source files)
- src/site/xdoc/docs/xmpp-transport.xml
- Dependencies from distribution, webapp, and apidocs POMs
- Module declaration from parent POM
- TOC entry from site docs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: robertlazarski

apidocs/pom.xml

@@ -194,11 +194,7 @@
             <artifactId>axis2-transport-udp</artifactId>
             <version>${project.version}</version>
         </dependency>
-        <dependency>
-            <groupId>${project.groupId}</groupId>
-            <artifactId>axis2-transport-xmpp</artifactId>
-            <version>${project.version}</version>
-        </dependency>
+        <!-- AXIS2-6106: axis2-transport-xmpp removed -->
         <!-- Artifacts that are needed to generate the Javadoc but that are not transitive dependencies
              because they are in scope "provided". -->
         <dependency>

modules/distribution/pom.xml

@@ -214,25 +214,7 @@
             <artifactId>axis2-transport-udp</artifactId>
             <version>${project.version}</version>
         </dependency>
-        <dependency>
-            <groupId>${project.groupId}</groupId>
-            <artifactId>axis2-transport-xmpp</artifactId>
-            <version>${project.version}</version>
-            <exclusions>
-                <exclusion>
-                    <groupId>jivesoftware</groupId>
-                    <artifactId>smack</artifactId>
-                </exclusion>
-                <exclusion>
-                    <groupId>jivesoftware</groupId>
-                    <artifactId>smackx</artifactId>
-                </exclusion>
-                <exclusion>
-                    <groupId>commons-lang</groupId>
-                    <artifactId>commons-lang</artifactId>
-                </exclusion>
-            </exclusions>
-        </dependency>
+        <!-- AXIS2-6106: axis2-transport-xmpp removed -->
 
         <!-- None of the Axis2 artifacts should have a dependency on log4j
              because we don't want to impose a particular logging implementation