Remove references to artifactory

[joker234]

All public releases should be available in the central repository, starting with the next release (see #181). Documentation should be changed with the release to remove the additional heigit release repository. If changed earlier, people would think this is already the case for the current release.

One example is:

oshdb/documentation/first-steps/README.md

Lines 32 to 41 in 540ee31

	```xml
	<repository>
	<snapshots>
	<enabled>false</enabled>
	</snapshots>
	<id>oshdb-releases</id>
	<name>Heigit/GIScience repository (releases)</name>
	<url>http://repo.heigit.org/artifactory/libs-release-local</url>
	</repository>
	```

[SlowMo24]

off topic: can we get an image-icon here: https://mvnrepository.com/search?q=oshdb
?

[joker234]

off topic: can we get an image-icon here: https://mvnrepository.com/search?q=oshdb
?

I think this is a custom icon choice by the websites maintainer. I ask him if its possible to get icons to custom domains.