stm32.cfg: remove reset_config

Here's a patch for the double-reset problem on STM32.  I've tested
downloading and debugging with GDB and Eclipse, and everything seems
to work fine.

This effectively sets reset_config to none. trst_only would also
be ok, but that's better left to a board configuration file since
not all boards wire it up.

The NVIC is used to trigger reset, which at least on this chip also
pulses nSRST so the whole system does get rest -- exactly once.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>