removing exception handling in do_mkdir

This change is being made to allow callers to handle
any exception thrown by os.mkdir.

This function is currently never called anywhere in the code.
It was introduced as part of the first commit to this project
but it was never used.

This patch (http://review.gluster.org/#/c/5304/) removed the
early version of this function, and this patch
(http://review.gluster.org/#/c/5305/) added it back with new
exception handling.

Change-Id: I71325660cb47594b0804da3da21920e26d2055f2
Signed-off-by: Thiago da Silva <thiago@redhat.com>
Reviewed-on: http://review.gluster.org/7350
Reviewed-by: Prashanth Pai <ppai@redhat.com>
Reviewed-by: Chetan Risbud <crisbud@redhat.com>
Tested-by: Chetan Risbud <crisbud@redhat.com>