MQTT: remove standalone MQTT clien generator code.
Code was inherited from OPC-UA extension, but not properly maintained.
<xsd:attribute name="Command" type="xsd:string" use="optional" default="make -C %(buildpath)s -f ../project_files/Makefile all BEREMIZSRC=%(src)s BEREMIZCFLAGS=%(cflags)s MD5=%(md5)s USE_BEREMIZ=1 FROM_BEREMIZ=1"/>