Remove unused directive

We used to use a local.bash to set up our environment for building Go
for akaros.  Now everything is done thorugh a different setup, so this
if statement in this file is unneeded anymore.  It was basically a noop
now anyway.
Cleanup childfdmap stuff a little.

Now pull the childfdmap struct from the c header file instead of
duplicating it manually.  Will make it easier to catch bugs in the
future if the interface changes (plus the more automated, the better).