[C] Remove command block from invalidBlocks list. Fixes BUKKIT-4342

The Issue:

Pick block on command blocks does't work on craftbukkit but does on vanilla. Behavior is not intended.

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.


JIRA Ticket:

BUKKIT-4342 https://bukkit.atlassian.net/browse/BUKKIT-4342