Revert "Generate 4096 bits RSA keys"
Revert "Generate 4096 bits RSA keys"

This reverts commit 66e846982494301ead72e6e940c0ed496fecb340.

See PRO-4851 for more discussion. 4096 bit RSA keys are about 4x slower than 2048, and this gets _really slow_ when the tests are run under valgrind:

time valgrind t_uptane
4096 => 4m33,349s
2048 => 0m28,607s
1024 => 0m12,138s

advancedtelematic/sota_client_cpp,advancedtelematic/aktualizr,advancedtelematic/sota_client_cpp,advancedtelematic/aktualizr,advancedtelematic/aktualizr,advancedtelematic/aktualizr