Duplicate 0x2902 chacteristic descriptor prevents notify in web bluetooth

[agrath]

When the calling code declares a charactersitic descriptor with uuid 2902, and also declares the characteristic to be notify, bleno automatically adds the descriptor.
This doesn't break anything except web based bluetooth, where in Chrome, an unknown error is thrown while trying to subscribe to the chacteristic

I will submit a pull request that simply skips the 2902 descriptor being added if it is defined on the peripheral as it is already added by bleno (with more appropriate flag handling)