Bugfix: backup: Remove bandwidth display

This commit removes the bandwidth displayed during backup process. It is
misleading and seldom correct, because it's neither the "read
bandwidth" (only for the very first backup) nor the "upload bandwidth".
Many users are confused about (and rightly so), c.f. #1581, #1033, #1591

We'll eventually replace this display with something more relevant when
the new archiver code is ready.

https://github.com/restic/restic/pull/1595