Revert an ill-considered portion of my patch of 12-Mar, which tried to save a

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.