Drop enforcer rule usage

It is used to enforce Maven 3.9 but used Tycho version itself requires
3.9.6 so this check is actually pointless and it's best to let Tycho
define the requirement for Maven.

Author: akurtakov

org.eclipse.swtchart.cbi/pom.xml

@@ -99,26 +99,6 @@
   PLUGINS
     -->
     <plugins>
-      <plugin>
-        <groupId>org.apache.maven.plugins</groupId>
-        <artifactId>maven-enforcer-plugin</artifactId>
-        <version>3.6.1</version>
-        <executions>
-          <execution>
-            <id>enforce-maven</id>
-            <goals>
-              <goal>enforce</goal>
-            </goals>
-            <configuration>
-              <rules>
-                <requireMavenVersion>
-                  <version>3.9</version>
-                </requireMavenVersion>
-              </rules>
-            </configuration>
-          </execution>
-        </executions>
-      </plugin>
       <plugin>
         <groupId>${tycho.groupid}</groupId>
         <artifactId>tycho-packaging-plugin</artifactId>