watchdog: documentation: remove index-file

The 00-index file in the watchdog directory is, like many others,
outdated (conversion-howto is missing) and doesn't contain worthwhile
additional information. As it seems to be a maintenance burden without
much gain, simply remove it.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>