Clean up akka.build.version #26675 (#28415)

Since sbt-dynver now takes care of this and has successfully been tested with
the play and lagom nightly builds. If you really want to override the version
anyway, you can still add a 'version.sbt' and override 'version in ThisBuild'
and 'isSnapshot in ThisBuild' manually.

project/AkkaBuild.scala

@@ -27,47 +27,7 @@ object AkkaBuild {
 
   lazy val buildSettings = Def.settings(
     organization := "com.typesafe.akka",
-    Dependencies.Versions,
-    // use the same value as in the build scope
-    // TODO #26675 this can be removed once we use sbt-dynver instead of timestamps
-    // everywhere
-    version := (version in ThisBuild).value)
-
-  // TODO #26675 this can be removed once we use sbt-dynver instead of timestamps
-  // everywhere
-  lazy val currentDateTime = {
-    // storing the first accessed timestamp in system property so that it will be the
-    // same when build is reloaded or when using `+`.
-    // `+` actually doesn't re-initialize this part of the build but that may change in the future.
-    sys.props.getOrElseUpdate("akka.build.timestamp",
-      DateTimeFormatter
-        .ofPattern("yyyyMMdd-HHmmss")
-        .format(ZonedDateTime.now(ZoneOffset.UTC)))
-  }
-
-  // TODO #26675 this can be removed once we use sbt-dynver instead of timestamps
-  // everywhere
-  def akkaVersion: String = {
-    val default = "2.6-SNAPSHOT"
-    sys.props.getOrElse("akka.build.version", default) match {
-      case "timestamp" => s"2.6-$currentDateTime" // used when publishing timestamped snapshots
-      case "file" => akkaVersionFromFile(default)  
-      case v => v
-    }
-  }
-
-  // TODO #26675 this can be removed once we use sbt-dynver instead of timestamps
-  // everywhere
-  def akkaVersionFromFile(default: String): String = {
-    val versionFile = "akka-actor/target/classes/version.conf"
-    if (new File(versionFile).exists()) {
-      val versionProps = new Properties()
-      val reader = new FileReader(versionFile)
-      try versionProps.load(reader) finally reader.close()
-      versionProps.getProperty("akka.version", default).replaceAll("\"", "")
-    } else
-      default
-  }
+    Dependencies.Versions)
 
   lazy val rootSettings = Def.settings(
     Release.settings,