Fixed sapling drops from leaf blocks and refactured BlockCustomLeaves.

Removed leaf blocks from creative tab.
Also removed leaf block items, they can't be used so there is no need
for them.
Successfully tested all trees with /get tree <name>. Saplings drop
correctly.
Top leaf block of Dougles Fir is not being removed and the huge Acacia
tree is not being removed completely, leaving floating tree chunks.