Docs - remove note about SPANK_ prefix on environment variables.

This is no longer the case as of commit 224cd329 before 15.08
was released.

Bug 4228 / bug 1570.

doc/man/man8/spank.8

@@ -211,10 +211,6 @@ Functions are also available from within the \fBSPANK\fR plugins to
 establish environment variables to be exported to the Slurm
 \fBPrologSlurmctld\fR, \fBProlog\fR, \fBEpilog\fR and \fBEpilogSlurmctld\fR
 programs (the so-called \fBjob control\fR environment).
-The name of environment variables established by these calls will be prepended
-with the string \fISPANK_\fR in order to avoid any security implications
-of arbitrary environment variable control. (After all, the job control
-scripts do run as root or the Slurm user.).
 .LP
 These functions are available from \fBlocal\fR context only.
 .nf