Re-add isRemappedClass() check now that runtime deobf is enabled.

This reverts commit 107cddadfd0d6bb28715031be774850b50825253.
Update org.bukkit.Material to use deobf-aware Forge EnumHelper

Previously included its own addEnum(), which was not deobfuscation-aware,
causing java.lang.NullPointerException at org.bukkit.Material.addEnum(Material.java:734)
(full stack trace: https://gist.github.com/agaricusb/5273062) after enabling the
isRemappedClass fix from FML:

MinecraftPortCentral/MCPC-Plus@353d95d8632f9f872b82d709b28ec342346d9fdc

Now that the fix is re-enabled, we need to use Forge's EnumHelper to extend
the org.bukkit.Material enum with mod items. As part of this change, enum
replacement (descriptive names vs the numeric X#### names) is disabled until
a deobf-aware replacement for replaceEnum() can be found or developed.