<project xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
                        http://maven.apache.org/maven-v4_0_0.xsd">

    <modelVersion>4.0.0</modelVersion>
    <groupId>org.glyptodon.guacamole</groupId>
    <artifactId>guacamole-legacy-urls</artifactId>
    <packaging>jar</packaging>
    <version>0.9.9</version>
    <name>guacamole-legacy-urls</name>
    <url>http://guac-dev.org/</url>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <build>
        <plugins>

            <!-- Written for 1.6 -->
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <source>1.6</source>
                    <target>1.6</target>
                </configuration>
            </plugin>

        </plugins>
    </build>

</project>

<project xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
                        http://maven.apache.org/maven-v4_0_0.xsd">

    <modelVersion>4.0.0</modelVersion>
    <groupId>org.glyptodon.guacamole</groupId>
    <artifactId>guacamole-legacy-urls</artifactId>
    <packaging>jar</packaging>
    <version>0.9.9</version>
    <name>guacamole-legacy-urls</name>
    <url>http://guac-dev.org/</url>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

</project>

Remove unnecessary reference to build plugin.
Remove unnecessary reference to build plugin.