OpenZFS 6754 - zfs-tests: get_substr() function is redundant

OpenZFS 6754 - zfs-tests: get_substr() function is redundant

Authored by: Yuri Pankov <yuri.pankov@gmail.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>
OpenZFS-issue: https://www.illumos.org/issues/6754
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/01ff411
Closes #5787

Porting notes:
- Only the ACL test cases used this function and they have been
  dropped from ZoL since Linux used POSIX style ACLs.

  - functional/acl/nontrivial/zfs_acl_chmod_aclmode_001_pos.ksh
  - functional/acl/nontrivial/zfs_acl_chmod_inherit_003_pos.ksh
  - functional/acl/nontrivial/zfs_acl_chmod_rwx_002_pos.ksh