Fix kotlin-serialization plugins build scripts

1. There is no need to specify repositories.
2. We don't use 'org.jetbrains.kotlin:gradle-api:1.6' anymore.
3. kotlin-serialization-unshaded does not need dependencies,
because it does not contain source code (it simply packages jar).

libraries/tools/kotlin-serialization-unshaded/build.gradle

@@ -9,21 +9,6 @@ compileJava {
     options.fork = false
 }
 
-repositories {
-    mavenLocal()
-    jcenter()
-    maven { url 'http://repository.jetbrains.com/utils' }
-}
-
-dependencies {
-    compileOnly project(':kotlin-gradle-plugin-api')
-
-    compileOnly project(':kotlin-stdlib')
-    compileOnly project(path: ':kotlin-compiler-embeddable', configuration: 'runtimeJar')
-
-    compileOnly 'org.jetbrains.kotlin:gradle-api:1.6'
-}
-
 evaluationDependsOn(":kotlinx-serialization-compiler-plugin")
 
 jar {

libraries/tools/kotlin-serialization/build.gradle

@@ -9,20 +9,12 @@ compileJava {
     options.fork = false
 }
 
-repositories {
-    mavenLocal()
-    jcenter()
-    maven { url 'http://repository.jetbrains.com/utils' }
-}
-
 dependencies {
     compileOnly project(':kotlin-gradle-plugin')
     compileOnly project(':kotlin-gradle-plugin-api')
 
     compileOnly project(':kotlin-stdlib')
     compileOnly project(path: ':kotlin-compiler-embeddable', configuration: 'runtimeJar')
-
-    compileOnly 'org.jetbrains.kotlin:gradle-api:1.6'
 }
 
 evaluationDependsOn(":kotlinx-serialization-compiler-plugin")