Remove step number from getEmptyStep method

This MR removes the step number argument from the ChainConfigurationBase.getEmptyStep method (ATR-30694). This number was used to add a Step#_ prefix to the empty steps' name. However, any pre-existing prefix is overridden during the chain merging and menu alignment process, so there is no need for the manual prefix.

I compared the HLTMenu and HLTJobOptions json files, and there are no differences after this change.