Remove JCenter

All dependencies which were fetched from JCenter are now available via Maven Central. This source change is necessary becuase JCenter announced they werer going to be read-only starting at 31st March 2021 (https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter).

build.gradle

@@ -5,7 +5,6 @@ buildscript {
     repositories {
         google()
         mavenCentral()
-        jcenter()
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:7.0.3'
@@ -20,7 +19,6 @@ allprojects {
     repositories {
         google()
         mavenCentral()
-        jcenter()
         maven { url "https://jitpack.io" }
         maven { url "https://clojars.org/repo" }
     }