pgsql: Revert an ill-considered portion of my patch of 12-Mar, which

Log Message:
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Revert an ill-considered portion of my patch of 12-Mar, which tried to save a
few lines in sql_exec_error_callback() by using the function source string
field that the patch added to SQL function cache entries.  This doesn't work
because the fn_extra field isn't filled in yet during init_sql_fcache().
Probably it could be made to work, but it doesn't seem appropriate to contort
the main code paths to make an error-reporting path a tad faster.  Per report
from Pavel Stehule.

Modified Files:
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    pgsql/src/backend/executor:
        functions.c (r1.117 -> r1.118)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/functions.c.diff?r1=1.117&r2=1.118)