DepPriority: remove "rebuild" attribute

DepPriority: remove "rebuild" attribute

Since the addition of DepPriorityNormalRange and
DepPrioritySatisfiedRange in commit
bd369956b2a2fbc019a655a372628998499156c0, which solves most cases of
bug 199856, the Depriority.rebuild attribute doesn't appear to make any
difference. The edges that this attribute differentiates are already
naturally differentiated by the fact that the child node of a satisfied
buildtime dependency that's not being rebuilt will naturally be
identified as a leaf node earlier and removed from the graph, thereby
eliminating the edge before there's an opportunity to compare it with
a higher priority rebuild edge. The addition of the "optional"
attribute (in commit 15476805a156acd11fdaaa19212691e8ee09b309) also
plays a role here, since it converts some satisfied buildtime edges to
optional edges, thereby reducing their priority.