The combination of Net::Mac_address and
Genode::ascii_to(Net::Mac_address) required shaky quirks in several
places because GCC is not able to resolve the ascii_to overload if
base/xml_node.h was included to early. The current solution moves the
several ascii_to overloads "closer" to the Net types by putting them
into the Net namespace, where GCC reliably picks them up.
Hence, co-locating the ascii_to() utility with the overload type in the
same scope/namespace is good practice.
This patch removes the now obsolete <nic/xml_node.h> header file.
