Remove --usim-test command line option

Remove the --usim-test option: to my knowledge, it's not used by users, and is misleadingly placed in various places where there should not be a difference w.r.t. USIM configuration.

When making the changes, I assumed that --usim-test would be 0 (the default), which is the case in all executables when not specified in options except for the nr_dlsim, nr_pbchsmi, nr_ulsim simulators.

The only exception is the initialization of PDCP from the MAC, which does not seem to be correct (the PDCP is in the CU, so it does not make sense to call it from the MAC which is in the DU).