Remove obsolete prebuilts/clang/*/host/3.6 projects.

These haven't been used in years, but somehow remained in the
aosp/master manifest.
Change-Id: I65debb989468cee34fb1d7e925a3041edc38d9b8

default.xml

@@ -468,8 +468,6 @@
   <project path="platform_testing" name="platform/platform_testing" />
   <project path="prebuilts/android-emulator" name="platform/prebuilts/android-emulator" groups="pdk-fs" clone-depth="1" />
   <project path="prebuilts/build-tools" name="platform/prebuilts/build-tools" groups="pdk" clone-depth="1" />
-  <project path="prebuilts/clang/darwin-x86/host/3.6" name="platform/prebuilts/clang/darwin-x86/host/3.6" groups="pdk,darwin" clone-depth="1" />
-  <project path="prebuilts/clang/linux-x86/host/3.6" name="platform/prebuilts/clang/linux-x86/host/3.6" groups="pdk,linux" clone-depth="1" />
   <project path="prebuilts/clang/host/darwin-x86" name="platform/prebuilts/clang/host/darwin-x86" groups="pdk,darwin" clone-depth="1" />
   <project path="prebuilts/clang/host/linux-x86" name="platform/prebuilts/clang/host/linux-x86" groups="pdk" clone-depth="1" />
   <project path="prebuilts/devtools" name="platform/prebuilts/devtools" groups="pdk-fs" clone-depth="1" />