[krbdev.mit.edu #2634] Remove use of client principal from krb5_context (default_ccprincipal)

[Alexandra Ellwood via RT]

After discussing with the team we have decided to solve the problem by removing the 
default_ccprincipal field from the krb5_context and having KLL and Leash read the ccache to 
pull the principal (if any) out.  Keeping two pieces of information in sync when one can be 
derived from the other is more likely to cause problems than it is to solve anything.