COMMON: Revert "Fix mismatched new/delete in the MemoryPool class".

This reverts commit b585addc23de25c019f250986d50c0373219571d.

The allocation of MemoryPool's page memory is doing in MemoryPool::allocPage,
which uses ::malloc, thus we need to use ::free to free the page memory in
MemoryPool::freeUnusedPages.