Remove the sgetspent method from the pwd impl.

FreeBSD and OS X do not implement an equivalent to sgetspent, and the
implementation included before did not work as expected.

sgetspent is supposed to parse shadow user data in a string and return the first
entry. The implementation we had called getpwnam, with the
supplied string. This meant it was trying to find a user with a name equal to the
passed in shadow user data.

I tested the current implementation to see if there was some hidden functionality of getpwnam
that allowed it to parse it's input as shadow user data rather than just a name, but it just returns
null if you pass it anything that isn't the name of a user in the system

it's best to just remove the function all together.