Cleanup pom.xml

* Remove redundant pdf generation sections
* Remove targetDirectory variable, the default is fine
* Properly set webhelpDirname
* Remove cleanup section from Install and Image Guides that
  are now not needed anymore due to the above changes

Change-Id: I4a11d74d3e53738d33b8ba9b50119bb1c25a1aec
Merge "Cleanup pom.xml"