configure: Remove default NDK path value

This likely isn't always valid, and subverts auto-detection.

Author: alexcrichton

configure

@@ -625,7 +625,7 @@ valopt llvm-root "" "set LLVM root"
 valopt python "" "set path to python"
 valopt jemalloc-root "" "set directory where libjemalloc_pic.a is located"
 valopt build "${DEFAULT_BUILD}" "GNUs ./configure syntax LLVM build triple"
-valopt android-cross-path "/opt/ndk_standalone" "Android NDK standalone path (deprecated)"
+valopt android-cross-path "" "Android NDK standalone path (deprecated)"
 valopt i686-linux-android-ndk "" "i686-linux-android NDK standalone path"
 valopt arm-linux-androideabi-ndk "" "arm-linux-androideabi NDK standalone path"
 valopt aarch64-linux-android-ndk "" "aarch64-linux-android NDK standalone path"