buildstats: Drop disk data from buildstats

buildstats: Drop disk data from buildstats

The existing diskstats data from buildstats simply isn't useful. It
gives stats on the total IO counts on some random disk within the
system. This means that the count includes data from all other tasks
running at the same time and from any other process running on the
system.

I've been unable to find any use for the data so as a start at
reworking the class, remove the related code.

(From OE-Core rev: f9fb02909f332365cad329352956a29cff6eba77)

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