Remove --preserve-root from roll_aosp.sh

This flag is not supported on mac and generates errors when trying to
roll.

BUG=b:150892231

Change-Id: I22d10a2b9b37610d409df4d72e0aba2672f7b192
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2152666
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>

scripts/roll_aosp.sh

@@ -20,7 +20,7 @@ deps=(
 
 # Delete dep directories so that gclient can check them out
 for dep in ${deps[@]}; do
-    rm -rf --preserve-root $dep
+    rm -rf $dep
 done
 
 # Sync all of ANGLE's deps so that 'gn gen' works
@@ -69,7 +69,7 @@ git add Android.bp
 # Delete the .git files in each dep so that it can be added to this repo. Some deps like jsoncpp
 # have multiple layers of deps so delete everything before adding them.
 for dep in ${deps[@]}; do
-    rm -rf --preserve-root $dep/.git
+    rm -rf $dep/.git
 done
 
 for dep in ${deps[@]}; do