openblas: Remove outdated comment

I made that comment ages ago when OpenBLAS did download an LAPACK
on-demand during build. This is no longer the case as far as I can tell.
openblas: 0.2.9rc2

Going to a RC here (exceptionally) because on newer Macs (I tested on
two, one haswell and one Mac Pro) the CPU auto detection fails and
formula install aborts. Also if we would force a certain TARGET, the
logic for that would be quite complicated. Further, the test did not
pass on a haswell based system. I hope this does not break other
software Homebrew/science.