usual.m4: remove libevent 1.3b check, it's too old
usual.m4: remove libevent 1.3b check, it's too old

And it's based on evhttp detection, which may not
be available.

ifduyue/libusual,rabbitt/libusual,mgjv/libusual,ifduyue/libusual,markokr/libusual,rabbitt/libusual,mgjv/libusual,mgjv/libusual,rabbitt/libusual,markokr/libusual,markokr/libusual,rabbitt/libusual,ifduyue/libusual,mgjv/libusual,markokr/libusual,ifduyue/libusual