Remove unused dict BYTE_MULTIPLIERS

Dict BYTE_MULTIPLIERS in ironic/common/utils.py isn't used
anywhere, and ironic/openstack/common/strutils.py includes
this dict, So we can use the one in oslo if we need. It's
safe to remove it now.

Change-Id: Iff96369057741c9a5b97fe44be7e6dd77c21ee82
Merge "Remove unused dict BYTE_MULTIPLIERS"