There's a long discussion in another post as to the best way to disable ULX motd.
Remove the file addons/ulx/lua/modules/cl/motdmenu.lua and make sure your gmod/data/ulx/config.txt doesn't have motdfile set to a ulx motd.
No editing is ever recommended for ULX files, as those two steps should properly allow any other motd system to work.
But, to answer your question, the file is lua/ulx/modules/sh/menus.lua - BUT, again, the motd function in that file checks for existence of the motdmenu.lua file, AND for a boolean variable set within the motdmenu.lua file.