PFWScriptGenerator: remove incomplete and unused XML output support

This script is used to generate XML files through the parameter-framework's
remote interface. A "direct xml output" support was partially implemented to
generate the XML output without the involvement of the parameter-framework.

It has never been comletely implemented nor used by anyone, so we'd better
remove it.

Change-Id: I6ad166306585c55884482ed03934da9f3efc70df
Signed-off-by: David Wagner <david.wagner@intel.com>