Remove duplicated documentation

'assumptions.md' contains similar contents to the other files in the
doc/cprover-manual folder.  The version in the other files is more
up-to-date.  This commit removes the duplicate to avoid confusion when
using or updating the documentation.

Also, this removes the section "Bounded Model Checking with CBMC"
from cbmc-tutorial.md, because there is an idential copy of that section
in  introduction.md.

Author: John Nonweiler