Remove maven-javadoc-plugin memory settings from base pom.xml

When attempting to build the Kernel site from trunk I encountered a build failure when trying to generate JavaDocs. Aaron Z. noted that the following settings often cause build failures and should be removed in favour of MAVEN_OPTS settings (which in my case exist).

So, remove these settings from the base pom.

<maxmemory>512m</maxmemory>
<additionalJOption>
-J-Xms168m -J-Xmx512m -J-XX:PermSize=128m -J-XX:NewSize=64m
</additionalJOption>