Remove redundant initialization of smgr pointer for relcache

Hi~

We seem to have a reduntant initialization of rd_smgr for a newly created
relcache entry. Commonly, the pointer is assigned as NULL after a relcache
is palloc'ed. But this one seems unnecessary because the rd_smgr has already
been initialized by earlier AllocateRelationDesc. Propose a patch for it.

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Regards, Jingtang