Remove version files in zookeeper-server/src/main from .gitignore

The Info.java and VersionInfoMain.java files are currently generated into the target/generated-sources directory. 

However .gitignore still includes the following lines for the main src directory.

zookeeper-server/src/main/java/org/apache/zookeeper/version/Info.java
zookeeper-server/src/main/java/org/apache/zookeeper/version/VersionInfoMain.java

Let's remove them.

I've just spent two hours trying to debug  mysterious build failures, which were caused by an old Info.java file in src, which didn't show up in git status because of those out-of-date .gitignore entries.