FreeBSD: Remove unused SECLABEL code

[ghost]

Motivation and Context

SECLABEL is undefined on FreeBSD.

Description

Prune ifdef SECLABEL code.

How Has This Been Tested?

Compiled

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[codecov]

Codecov Report

Merging #10847 into master will decrease coverage by 0.36%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10847      +/-   ##
==========================================
- Coverage   79.88%   79.51%   -0.37%     
==========================================
  Files         395      395              
  Lines      125066   125066              
==========================================
- Hits        99905    99445     -460     
- Misses      25161    25621     +460     

Flag	Coverage Δ
#kernel	80.31% <ø> (-0.08%)	⬇️
#user	64.47% <ø> (-1.48%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/zdb/zdb_il.c	30.86% <0.00%> (-24.08%)	⬇️
module/os/linux/spl/spl-kmem-cache.c	78.44% <0.00%> (-11.64%)	⬇️
cmd/ztest/ztest.c	76.20% <0.00%> (-5.91%)	⬇️
cmd/zvol_id/zvol_id_main.c	76.31% <0.00%> (-5.27%)	⬇️
module/zfs/vdev_indirect.c	73.37% <0.00%> (-3.83%)	⬇️
module/zfs/vdev_removal.c	94.05% <0.00%> (-2.63%)	⬇️
module/zfs/vdev_indirect_mapping.c	97.10% <0.00%> (-1.94%)	⬇️
module/zfs/zio_compress.c	92.72% <0.00%> (-1.82%)	⬇️
module/zfs/metaslab.c	93.93% <0.00%> (-1.81%)	⬇️
module/zfs/vdev_queue.c	94.69% <0.00%> (-1.77%)	⬇️
... and 51 more

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