Is the app "SIM tools kit" important
According to the information I can find, the SIM toolkit is act as an interface to the cellular provider's services. Removing this will likely break phone's cellular capabilities, however I cannot confirm this as I do not want to risk deleting it on my device.

Because this toolkit is dependent on the SIM card itself and not the cellular provider, you will need to look up the source code and/or licensing for each SIM manufacturer individually.