scripts: remove zephyr_flash_debug.py

Since, by design, the west 'flash', 'debug', and 'debugserver'
commands operate the same way as the Makefile targets when given no
arguments, so just delegate to west from cmake/flash/CMakeLists.txt
instead of invoking zephyr_flash_debug.py by hand.

Removing the old script is the first step towards being able to clean
up the command line argument handling in the runner package, which was
always kind of a hack and can be improved now that runner is part of a
larger tool.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>