From the third line of that file you can figure out where the code is
for your INLA installation:
> system.file("demodata/Bym-map.R", package="INLA")
[1] "/usr/local/lib/R/site-library/INLA/demodata/Bym-map.R"
(Bym.map() is a trivial wrapper for germany.map() which is also
defined in that file.)
/Finn