pwm.9 symlink shouldn't be removed

When the pwm.9 manual was removed, a symlink between pwmbus.9 and pwm.9 was
created, but there's an entry in ObsoleteFiles.inc to remove pwn.9, meaning
that on every installation pwm.9 is created, and make delete-old deletes it.

Remove the entry from ObsoleteFiles.inc, the symlink is clearly intentional
and shouldn't be removed.

Reviewed by: imp, ian
Approved by: imp (implicit, review OK)
Differential Revision: https://reviews.freebsd.org/D21198