[symbology] fix duplicate layers in symbol levels dialog

[nirvn]

Description

This PR fixes a regression following commit a899963, whereas symbol layers are shown twice in the symbol levels dialog.

It fixes part of the issue filed here: https://issues.qgis.org/issues/16996 -- however, it still doesn't address the bigger issue, namely: symbol levels are broken, always set to 0!

@wonder-sk , I suspect the regression might be linked to your legend rework / cleanup. Could you have a look at this PR, as well as the bigger issue raised? Cheers.

Checklist

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[nirvn]

@wonder-sk , I'm going to merge this now. Hope you can take a look at the more serious regression in the near future (i.e. symbol lelvels not working for rule-based symbology).