testimage: Drop target_dumper and most of monitor_dumper

testimage: Drop target_dumper and most of monitor_dumper

The target_dumper code is basically broken. It has been reading binary files
over the text base serial communication and runs at every command failure which
makes no sense. Each run might overwrite files from the previous run and the
output appears corrupted due to confusion from the binary data.

For now, remove the commands and the target dumper code as the command
and execution point are problematic. Also remove the same pieces of the monitor
code but leave the command list since in theory this can be moved to a more
useful place in the code.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>