qibuild: remove opt-depend of lxml, now useless
qibuild: remove opt-depend of lxml, now useless

Also, this fixes qixml.write() when lxml is installed.

Change-Id: I97573be1b3cc5db4bd5cb6942b7a3402b0414b77

aldebaran/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild