Can the '--set-upstream' option of branch be removed ?

I refactored builtin/branch.c to remove the '--set-upstream'
option,successfully. The corresponding patch follows. 

There's just one issue with the version of git that doesn't
have the '--set-upstream' option. It's described in the commit
log message of the following patch.

I guess it would be difficult to detect the removal of the option in
case it's used in scripts and might cause confusion to users?
Is it ok to proceed with the removal?

BTW, It's now clear  to me that removing '--set-upstream' has nothing
to do with merging the two parameter of 'validate_new_branchname'.

-- 
Kaartic