Remove canShare method?

[mgiuca]

As discussed with @marcoscaceres yesterday, the canShare method may be unnecessary.

Since it provides no actual arguments to query exactly what actions are supported, and any implementation of Web Share is likely to have at least one default action, it's likely that canShare would simply always return true in any implementation. Therefore, people would not check it and it would probably break sites if a rare implementation returned false. So we might just drop it (and sites can use the presence of navigator.share itself to detect whether sharing is supported).

On the other hand, if we are going to share more interesting types of data (like files, images, etc), there may be different sets of apps that can handle these data types, and we may want canShare to take an argument that describes the data, to see if there is an app that can handle that particular type.