python: Remove unused debug APIs

python: Remove unused debug APIs

The following APIs are not exported from the _m5 namespace and not
used by any of the debug glue code:

 * m5.debug.findFlag
 * m5.debug.setDebugFlag
 * m5.debug.clearDebugFlag
 * m5.debug.dumpDebugFlags

All of them have a clean Python interface where flags are exported
using the m5.debug.flags dictionary. There is also an m5.debug.help
function that lists the available debug flags.

Remove the unused APIs to avoid confusion.

Change-Id: I74738451eb5874f83b135adaccd30a0c6b478996
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34120
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>