Back out timegm error check from r272562.

POSIX treats negative time_t as undefined (i.e. may be valid too,
depends on system's policy we don't have) and we don't set EOVERFLOW
in mktime/timegm as POSIX requires to surely distinguish -1 return
as valid negative time from -1 as error return.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272678 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f