Remove prebuilts/ninja/*

We put ninja prebuilts in prebuilts/build-tools now instead.
Test: Remove locally, try a build.
Test: Searched the code for other references.

default.xml

@@ -490,8 +490,6 @@
   <project path="prebuilts/maven_repo/android" name="platform/prebuilts/maven_repo/android" groups="pdk-cw-fs,pdk-fs" clone-depth="1" />
   <project path="prebuilts/misc" name="platform/prebuilts/misc" groups="pdk" clone-depth="1" />
   <project path="prebuilts/ndk" name="platform/prebuilts/ndk" groups="pdk" clone-depth="1" />
-  <project path="prebuilts/ninja/darwin-x86" name="platform/prebuilts/ninja/darwin-x86" groups="darwin,pdk,tradefed" clone-depth="1" />
-  <project path="prebuilts/ninja/linux-x86" name="platform/prebuilts/ninja/linux-x86" groups="linux,pdk,tradefed" clone-depth="1" />
   <project path="prebuilts/python/darwin-x86/2.7.5" name="platform/prebuilts/python/darwin-x86/2.7.5" groups="darwin,pdk,pdk-cw-fs,pdk-fs" clone-depth="1" />
   <project path="prebuilts/python/linux-x86/2.7.5" name="platform/prebuilts/python/linux-x86/2.7.5" groups="linux,pdk,pdk-cw-fs,pdk-fs" clone-depth="1" />
   <project path="prebuilts/qemu-kernel" name="platform/prebuilts/qemu-kernel" groups="pdk" clone-depth="1" />