All AST nodes are allocated in a memory pool and get deleted by the pool.
RemoveTree doesn't do anything because delete is overloaded for the nodes.

BUG=18469191

Change-Id: I1dc23e894c441db2e2eb1f6e9b8f0c11b2e52b3c
Reviewed-on: https://swiftshader-review.googlesource.com/1452Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>