Revert "Speed up ndk-build from sbt-android"

.travis.yml

@@ -50,4 +50,4 @@ install:
       fi
 
 script:
-    - $SBTPATH/sbt -DNPROC=$(nproc) native-build android:package-debug
+    - $SBTPATH/sbt native-build android:package-debug

build.sbt

@@ -8,11 +8,6 @@ name := "shadowsocks"
 version := "3.4.1"
 versionCode := Some(173)
 
-// Speed up ndk-build by reading nproc from environment
-// and default to 8
-val nproc = sys.props.getOrElse("NPROC", default = "8")
-ndkArgs := Seq(s"-j$nproc")
-
 platformTarget := "android-25"
 
 compileOrder := CompileOrder.JavaThenScala