Use computePhaseDescriptors directly

This commit does two things:
* In 2.8, `computePhaseDescriptors` was accessed using reflection
  because it was private and inaccessible to `CachedCompiler`.
  However, it was turned into protected later on by scala/scala@1016d68.
  As current Zinc only supports Scala versions 2.10, 2.11 and 2.12, the
  access to `computePhaseDescriptors` is no longer reflective.
* Remove unused imports that cluttered the source file.

internal/compiler-bridge/src/main/scala/xsbt/CompilerInterface.scala

@@ -9,15 +9,9 @@ package xsbt
 
 import xsbti.{ AnalysisCallback, Logger, Problem, Reporter, Severity }
 import xsbti.compile._
-import scala.tools.nsc.{ backend, io, reporters, symtab, util, Phase, Global, Settings, SubComponent }
-import scala.tools.nsc.interactive.RangePositions
-import backend.JavaPlatform
-import scala.tools.util.PathResolver
-import symtab.SymbolLoaders
-import util.{ ClassPath, DirectoryClassPath, MergedClassPath, JavaClassPath }
-import ClassPath.{ ClassPathContext, JavaContext }
+import scala.tools.nsc.{ io, reporters, Phase, Global, Settings, SubComponent }
+import scala.tools.nsc.util.ClassPath
 import io.AbstractFile
-import scala.annotation.tailrec
 import scala.collection.mutable
 import Log.debug
 import java.io.File
@@ -223,8 +217,7 @@ private final class CachedCompiler0(args: Array[String], output: Output, initial
         }
         superComputePhaseDescriptors
       }
-    // Required because computePhaseDescriptors is private in 2.8 (changed to protected sometime later).
-    private[this] def superComputePhaseDescriptors() = superCall("computePhaseDescriptors").asInstanceOf[List[SubComponent]]
+    private[this] def superComputePhaseDescriptors() = this.computePhaseDescriptors
     private[this] def superDropRun(): Unit =
       try { superCall("dropRun"); () } catch { case e: NoSuchMethodException => () } // dropRun not in 2.8.1
     private[this] def superCall(methodName: String): AnyRef =