I'm with Tommays, seal it up and forget about it. It's very unlikely to cause problems.
If you absolutely want it out, look for a core drill of slightly larger size than the tap and use it to core the drill out. Drill to the correct dpth, then use a small chisel to pop the core side to side, it will break the core out and you can then lift the core and tap out. Of course, then you have a hole in the end of the bolt which will trap water, so you'd need to fill/seal it anyway so it's just as easy quicker to seal it with the tap in it.