Can I eliminate RNase A from buffer P1 for my plasmid preparation to obtain RNase-free DNA for in-vitro transcription?

No, RNase A should not be omitted from buffer P1. It is required to prevent RNA contamination of the purified plasmid DNA. RNase A will not interfere with downstream in-vitro transcription experiments, since it will be efficiently removed during the plasmid purification procedures using QIAGEN Plasmid Kits.