s3/winbindd: Remove unnecessary check for NULL SID

s3/winbindd: Remove unnecessary check for NULL SID

There's a known bug in some Windows implementations of
DsEnumerateDomainTrusts() where domain SIDs are not returned for
transitively trusted domains within the same forest.

Jerry originally worked around this in the winbindd parent by checking
for S-0-0 and converting it to S-1-0 in 8b0fce0b.  Guenter later moved
these checks into the child process in commit 3bdfcbac making the
initial patch unecessary.

I've removed it and added a clarifying comment to the child process.

If ever this SID is needed we could add an extra DsEnumerateDomainTrusts()
call in trusted_domains() as suggested by the Microsoft KB.