Revert explicit check for building with Java 1.6

#KT-2988 Fixed

libraries/pom.xml

@@ -124,31 +124,6 @@
                     </systemProperties>
                 </configuration>
             </plugin>
-            <!--
-              the build doesn't work in JDK 7 yet so lets enforce 1.6 with meaningful errors to avoid confusion
-               see: http://youtrack.jetbrains.com/issue/KT-2988 for more details
-              -->
-            <plugin>
-              <groupId>org.apache.maven.plugins</groupId>
-              <artifactId>maven-enforcer-plugin</artifactId>
-              <version>1.0.1</version>
-              <executions>
-                <execution>
-                  <id>enforce-versions</id>
-                  <phase>validate</phase>
-                  <goals>
-                    <goal>enforce</goal>
-                  </goals>
-                  <configuration>
-                    <rules>
-                      <requireJavaVersion>
-                        <version>[1.6,1.7)</version>
-                      </requireJavaVersion>
-                    </rules>
-                  </configuration>
-                </execution>
-              </executions>
-            </plugin>
         </plugins>
     </build>
 </project>