Revert 249835 "Add fallback support for custom serial baud rates..."

This has created more problems than it has resolved.

> Add fallback support for custom serial baud rates on OS X.
> 
> Many drivers do not support the IOSSIOSPEED ioctl, but do
> support arbitrary baud rates. This adds a fallback for cases
> where IOSSIOSPEED fails.
> 
> BUG=337482
> TEST=manual
> 
> Review URL: https://codereview.chromium.org/137653005

TBR=rockot@chromium.org

Review URL: https://codereview.chromium.org/137433014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249955 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249955