lib1500: remove bad check

After curl_multi_wait() returns, this test checked that we got exactly
one file descriptor told to read from, but we cannot be sure that is
true. curl_multi_wait() will sometimes return earlier without any file
descriptor to handle, just just because it is a suitable time to call
*perform().

This problem showed up with commit 29bf0598.

Bug: http://curl.haxx.se/mail/lib-2013-06/0029.html
Reported-by: Fabian Keil