Is removal of the DMSO from the HEK293T cells required?

Yes, it is absolutely required. You cannot simply take the cells out of the cryovial and put them into 10ml media in a flask. There is a very high percentage of DMSO in the cryofreezing media. A 1:10 dilution is not enough to eliminate the immense toxicity of the DMSO. You must take the cells into 10ml fresh medium out of the cryovial, mix, then spin, aspirate supernatant (removing DMSO), resuspend and put onto the flask. This is all list in the protocol for culturing the HEK293T cells included in the Trans-Lentiviral Packaging System product manual.