Some arc_release() cleanup

[amotin]

• Don't drop L2ARC header if we have more buffers in this header. Since we leave them the header, leave them the L2ARC header also. Honestly we are not required to drop it even if there are no other buffers, but then we'd need to allocate it a separate header, which we might drop soon if the old block is really deleted. Multiple buffers in a header likely mean active snapshots or dedup, so we know that the block in L2ARC will remain valid. It might be rare, but why not?
• Remove some impossible assertions and conditions.

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)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS 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.