Revert "java: Publish to GitHub Packages instead of Maven Central"

This reverts commit b2c5685080.

java/server/build.gradle

@@ -85,16 +85,6 @@ processResources {
 // MARK: Publishing
 
 publishing {
-    repositories {
-        maven {
-            name = "GitHubPackages"
-            url = uri("https://maven.pkg.github.com/signalapp/libsignal")
-            credentials {
-                username = project.findProperty("githubUsername")
-                password = project.findProperty("githubToken")
-            }
-        }
-    }
     publications {
         mavenJava(MavenPublication) {
             artifactId = archivesBaseName