Remove BountySource from funding sources

Nobody has used it recently, and it seems to be in a downwards spiral,
with the escrow stealing announcement.

.github/FUNDING.yml

@@ -1,2 +1 @@
 liberapay: Cuberite
-custom: https://salt.bountysource.com/teams/cuberite

BACKERS

@@ -17,6 +17,5 @@ Thanks to the following people for supporting the Cuberite project with a donati
  - Utking
  - VaiN474
 
-If you enjoy Cuberite, feel free to donate to the project on Liberapay or Bountysource:
+If you enjoy Cuberite, feel free to donate to the project on Liberapay:
 https://liberapay.com/Cuberite
-https://salt.bountysource.com/teams/cuberite

README.md

@@ -66,7 +66,7 @@ You can also help with documentation by contributing to the [User's Manual][12].
 
 For other stuff, check out the [homepage][13], the [Users' Manual][14], the [forums][15], and the [Plugin API][16].
 
-Support Us on [Liberapay][17] or [Bountysource][18]
+Support the Cuberite development team on [Liberapay][17]
 
 [1]: https://cuberite.org/news/#subscribe
 [2]: https://cuberite.org/
@@ -84,4 +84,3 @@ Support Us on [Liberapay][17] or [Bountysource][18]
 [15]: https://forum.cuberite.org/
 [16]: https://api.cuberite.org/
 [17]: https://liberapay.com/Cuberite
-[18]: https://bountysource.com/teams/cuberite