Remove redundant effort to drop privileges during editing

This is an odd artifact from 2003.

crontab creates a temporary file and then forks an editor to edit that
file. Before this happens, privileges must be dropped, because crontab
is SGID "crontab".

Before the fork, there was a call to swap_uids(), which temporarily
drops the privileges. After the editor has returned, the privileges are
reasumed with swap_uids_back().

However, after forking but before exec'ing the editor, the child
also drops privileges.

The swap_uid() and swap_uid_back() therefore only affect the parent
process, but all the parent does in between this calls is to wait for
the client to exit.