testimage: drop target_dumper and host_dumper

testimage: drop target_dumper and  host_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.

It isn't possible to cherry-pick "testimage: Drop target_dumper and most of monitor_dumper"
from master, so just make target_dumper and host_dumper empty functions.

For further details see:

https://lists.openembedded.org/g/openembedded-architecture/message/1888

Signed-off-by: Steve Sakoman <steve@sakoman.com>