Remove unused nova.crypto.compute_md5()

Remove nova.crypto.compute_md5() as it is:
 * unused
  * last user removed 3 years ago with commit bab3060
 * untested
 * incorrectly documented
  * return type is not a tuple, but a string
 * redundant
  * copied from boto which is a nova requirement
 * burdened with license text more verbose than associated code

Change-Id: Ia033d3359098fc0af8de00248dc14b4fe280e844
Merge "Remove unused nova.crypto.compute_md5()"