OpenZFS 9190 - Fix cleanup routine in import_cachefile_device_replaced.ksh

OpenZFS 9190 - Fix cleanup routine in import_cachefile_device_replaced.ksh

Must clear slow-disk zinject injections in test cleanup routine.
Otherwise, when this test fails, it causes most subsequent tests
to fail.

Authored by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://illumos.org/issues/9190
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/762c6b4
Closes #7530