Remove the certificate password code from the client

[oyvindhagberg]

❗ This has to wait until we're sure that no clients still use old certificate files with passwords.

• Remove the subroutine getpassword and re-write the code that uses it. It deals with certificate key files that are password-protected, this was done to support an older server version that isn't in use anymore. Newer certificates are not password protected.
• There's a test in tests/test_cert_handling.sh that has to be removed.