Use this:
mChart.getAxisLeft().setDrawGridLines(false);
mChart.getXAxis().setDrawGridLines(false);
Please note you may need right axis or both of them. It depends on axis you are actually using.
UPDATE:
Is it axis line? If it is, then simply chart.getXAxis().setEnabled(false)
Also possible: chart.getAxisLeft().setDrawAxisLine(false)
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