Fix slowness in debug run of test-tcp-many-clients

I'm not sure what is actually causing

  node_g test/mjsunit/test-tcp-many-clients.js

to run slowly, but I traced the problem to
afd9e714.
Somehow it has to do with promoting the compilation of src/util.js. That
change wasn't actually intended to be included in afd9e7 commit anyway, so
I'm reverting it here.