Remove .xmobarrc as it is no longer used.

install/user.sh installs .xmobarrc.$(hostname) instead.
install/packages.txt: Add packages

 - rxvt-unicode-256color: Terminal emulator for virtual desktop 1
 - pulseaudio: Allows per-application volume control
 - pavucontrol: To control volume in pulseaudio
.xmobarrc.oxygen: Adjust thickness to fit trayer