rfc15: IMP input no longer uses stdin

Problem: the RFC states that the IMP takes its input on stdin
to avoid placing sensitive data on the command line, but stdin
is no longer used for this.

Now the IMP obtains its input by calling a helper program provided
by the instance instead of stdin.  The helper is run from the
unprivileged part of the IMP.

For now, just drop the incorrect detail which wasn't necessary
in that part of the text anyway.

See also: flux-framework/flux-security#163

spec_15.rst

@@ -122,8 +122,8 @@ design
    which MAY be installed with setuid permissions in cases where multi-user
    Flux is required.
 
--  The IMP SHALL accept and process data using stdin, to avoid putting
-   sensitive data on the command line or environment.
+-  The IMP SHALL avoid putting sensitive data on the command line or
+   environment.
 
 Implementation of the IMP as a separately installed, setuid executable
 allows sysadmin control over where and how the IMP is enabled. If the