Do not depend on Shadow plugin in subprojects

Since buildSrc depends on the Shadow plugin, it's already in the
classpath of all build scripts

custom-dependencies/protobuf-relocated/build.gradle.kts

@@ -1,16 +1,5 @@
-
 import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
 
-buildscript {
-    repositories {
-        jcenter()
-    }
-
-    dependencies {
-        classpath("com.github.jengelman.gradle.plugins:shadow:${property("versions.shadow")}")
-    }
-}
-
 val baseProtobuf by configurations.creating
 val baseProtobufSources by configurations.creating
 
@@ -62,4 +51,4 @@ val prepareSources by task<Jar> {
 
 val clean by task<Delete> {
     delete(buildDir)
-}
+}