Move symbols for ExecMergeJoin's state machine into nodeMergejoin.c.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 7b464015577e6e7544a7aacf19ca55b271e740ac
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2010-12-31T03:12:40Z
Releases: 9.1.0
Move symbols for ExecMergeJoin's state machine into nodeMergejoin.c.

There's no reason for these values to be known anywhere else.  After
doing this, executor/execdefs.h is vestigial and can be removed.

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