Fix TOTP authentication

Summary:
Kolab used to depend on version 4 of the otphp library, which returns
its computed OTP codes as integers. Hence, the kolab_2fa plugin converts
the user input to int in order to facilitate comparisons using the
=== operator.

Starting with version 5, which is now bundled with Kolab, otphp returns
a string instead of an integer. Now the comparison is between an int
and a string, and thus consistently yields false. As a result, no
TOTP code is ever accepted.

Fix TOTP authentication by removing the now-obsolete conversion to
int.

Reviewers: machniak

Reviewed By: machniak

Differential Revision: https://git.kolab.org/D4370