pgbench: cleanup use of a "logfile" parameter

pgbench: cleanup use of a "logfile" parameter

There is no reason to have the per-thread logfile file pointer as a
separate parameter in various functions: it's much simpler to put it in
the per-thread state struct instead, which is already being passed to
all functions that need the log file anyway.  Change the callsites in
which it was used as a boolean to test whether logging is active, so
that they use the use_log global variable instead.

No backpatch, even though this exists since commit a887c486d5df of March
2010, because this is just for cleanliness' sake and the surrounding
code has been modified a lot recently anyway.