ci: drop flaky onarandombox maven repo (fix 503 worldedit resolution)

The mv-repo (repo.onarandombox.com) intermittently returns 503 in CI and
is not needed: Multiverse-Core is only a plugin.yml softdepend (no maven
dep), and worldedit-core 7.3.6 resolves from maven.enginehub.org.

build.gradle.kts

@@ -61,11 +61,6 @@ repositories {
         url = uri("https://repo.extendedclip.com/content/repositories/placeholderapi/")
     }
     
-    maven {
-        name = "mv-repo"
-        url = uri("https://repo.onarandombox.com/content/groups/public/")
-    }
-    
     maven {
         name = "dmulloy2-repo"
         url = uri("https://repo.dmulloy2.net/repository/public/")