pgsql: Remove unused 'printCost' field from ExplainState, and simplify

Log Message:
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Remove unused 'printCost' field from ExplainState, and simplify the code
accordingly (this field was always initialized to true). Patch from
Alvaro Herrera.

Modified Files:
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pgsql/src/backend/commands:
explain.c (r1.136 -> r1.137)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c.diff?r1=1.136&r2=1.137)