mokutil: remove unused int_to_b64()

[nfrayer]

static const char b64t[64] triggers compiler warning which in turn makes the build fail with -Werror=unterminated-string-initialization, so removing this string with the the unused int_to_b64() function which is the only function using this array.

[yselkowitz]

@nfrayer ack, this would fix F42FTBFS.

[nfrayer]

Hi @lcp , please review and merge at your earliest convenience, thanks.

[lcp]

Thanks for the patch!