Remove lib/libspl/include/sys/frame.h

The functionality provided by this header is not required by any
of the ZFS user space code. Minimal functionality was provided
in commit c28a677 which added include/sys/frame.h.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6960
Closes #6972