vfs: Remove incorrect debugging WARN in prepend_path

The warning message in prepend_path is unclear and outdated.  It was
added as a warning that the mechanism for generating names of pseudo
files had been removed from prepend_path and d_dname should be used
instead.  Unfortunately the warning reads like a general warning,
making it unclear what to do with it.

Remove the warning.  The transition it was added to warn about is long
over, and I added code several years ago which in rare cases causes
the warning to fire on legitimate code, and the warning is now firing
and scaring people for no good reason.

Cc: stable@vger.kernel.org
Reported-by: Ivan Delalande <colona@arista.com>
Reported-by: Omar Sandoval <osandov@osandov.com>
Fixes: f48cfddc ("vfs: In d_path don't call d_dname on a mount point")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>