[C] Brief summary. Fixes BUKKIT-4342

The Issue:

Paragraphs explaining what the issue the PR is meant to be addressing.

Justification for this PR:

Vanilla behavior of cloning the command block when using pick block in creative doesn't happen when using 
Craftbukkit server

PR Breakdown:

Simply removing the command block's item id from the list of invalid items

Testing Results and Materials:

I ran the server jar with and without the PR and attempted to use the pick block, the behavior only operated as expected when using this patch.

Here's a jar: https://www.dropbox.com/s/qy62zw43tv09yjj/craftbukkit-1.7.9-R0.3-SNAPSHOT.jar

JIRA Ticket:

BUKKIT-4342 https://bukkit.atlassian.net/browse/BUKKIT-4342