pad.c:pad_free: Don’t turn off the PADTMP flag

pad.c:pad_free: Don’t turn off the PADTMP flag

When we mark a pad entry as being free, it may happen (under
USE_BROKEN_PAD_RESET) that multiple operators from different state-
ments are using the same pad entry for their targets.  If we turn off
the PADTMP flag, the slot may be reused for a constant.  Then an oper-
ator that tries to return a value by assigning to its target will try
to modify a read-only scalar.

Now that (as of a few commits ago) allocation of targets (in
pad_alloc) will reuse entries marked PADTMP, turning off the flag is
not necessary to make a slot available.

This gets the build for USE_BROKEN_PAD_RESET+threads a little further.
The Exporter test now passes, but configpm trips an assertion....