Clear existing OSM markers before binding

[maniac103]

Makes sure markers of previous binding do not accumulate.

Fixes #3978

[maniac103]

@dilyanpalauzov Would you be able to build and test this?

[dilyanpalauzov]

I cannot build.

[maniac103]

I can build an APK, but it won't update an official version, so you'll need to uninstall that one before installing the APK. Is this OK for you nevertheless?

[dilyanpalauzov]

I could install that build, but I cannot actually test. The tracked item does not move currently, it might or might not move after a month. So I will have to send POST requests to openHAB to update the state of the Location item. I will have to do it several times, and if it shows the old behaviour - then the problem is not fixed. If it does not show the old behaviour, then maybe the problem is corrected, or maybe I am testing in a wrong way.

For these reasons I suggest to include the change in the next regular release, without testing from my side.