Stop setting net.hostname

Test: as follows
    - basics:
        - built
        - flashed
        - booted
    - connect to {v4-only, dualstack, v6-only} networks
        - verified expected connectivity on each
    - runtest frameworks-net
Bug: 30979015

Change-Id: I65e9878c7b8eaddaba1c0e1b8566ddfa0760047f