pgsql: Move symbols for ExecMergeJoin's state machine into nodeMergejoi

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.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=7b464015577e6e7544a7aacf19ca55b271e740ac

Modified Files
--------------
src/backend/executor/nodeMergejoin.c | 16 +++++++++++++++-
src/include/executor/execdefs.h | 33 ---------------------------------
src/include/nodes/execnodes.h | 2 +-
3 files changed, 16 insertions(+), 35 deletions(-)