remove useless check
remove useless check

This function is defined in `Helpers.mk`, so it was undefined here,
expanding to nothing, and doing nothing :O

The check will effectively be done by the includes that'll fail on
the next lines anyway (unless you've got Tock makefiles in `/` :D)

tock/libtock-c,tock/libtock-c,tock/libtock-c