Revert of Turn on clang by default on linux, 2nd try.

tools/clang/scripts/update.sh

Index: tools/clang/scripts/update.sh
diff --git a/tools/clang/scripts/update.sh b/tools/clang/scripts/update.sh
index e86c288875f2f1f3d6de4cdb3ac8b3fe529e9f9c..2beb4e39b28b32c1cfb3f6d1bcfa1582cc82d516 100755
--- a/tools/clang/scripts/update.sh
+++ b/tools/clang/scripts/update.sh
@@ -137,6 +137,12 @@
   shift
 done
 
+# Remove clang on bots where it was autoinstalled in r262025.
+if [[ -f "${LLVM_BUILD_DIR}/autoinstall_stamp" ]]; then
+  echo Removing autoinstalled clang and clobbering
+  rm -rf "${LLVM_BUILD_DIR}"
+fi
+
 if [[ -n "$if_needed" ]]; then
   if [[ "${OS}" == "Darwin" ]]; then
     # clang is used on Mac.
@@ -147,12 +153,6 @@
   elif [[ -d "${LLVM_BUILD_DIR}" ]]; then
     # clang previously downloaded, remove third_party/llvm-build to prevent
     # updating.
-    true
-  elif [[ "${OS}" == "Linux" ]]; then
-    # Temporarily use clang on linux. Leave a stamp file behind, so that
-    # this script can remove clang again on machines where it was autoinstalled.
-    mkdir -p "${LLVM_BUILD_DIR}"
-    touch "${LLVM_BUILD_DIR}/autoinstall_stamp"
     true
   else
     # clang wasn't needed, not doing anything.