Revert "bridge: Allow forward delay to be cfgd when STP enabled"

This reverts commit 34c2d9fb.

There are 2 reasons for this revert:
 1)  The commit in question doesn't do what it says it does.  The
     description reads: "Allow bridge forward delay to be configured
     when Spanning Tree is enabled."  This was already the case before
     the commit was made.  What the commit actually do was disallow
     invalid values or 'forward_delay' when STP was turned off.

 2)  The above change was actually a change in the user observed
     behavior and broke things like libvirt and other network configs
     that set 'forward_delay' to 0 without enabling STP.  The value
     of 0 is actually used when STP is turned off to immediately mark
     the bridge as forwarding.

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>