[infra] Remove depot_tools from infra manifest

We use our own versions of prebuilts like lucicfg and vpython rather
than using Chromium's depot_tools versions, and ~never need to make
depot_tools modifications, so it's not necessary to include depot_tools
in an infra checkout.

It's also confusing to include depot_tools results when running grep
from the root of an infra checkout, since the depot_tools repo contains
some recipes that can easily be mistaken for our own fuchsia recipes.

I assume the original reason for including depot_tools in the infra
checkout was to permit access to lucicfg and other executables, but I
can't say for sure because the CL that added the dependency doesn't
provide any motivation: http://fxrev.dev/302149

Change-Id: Ic89a02182b9440cb5734ab5bbde775c1255200ee
Reviewed-on: https://fuchsia-review.googlesource.com/c/manifest/+/458736
Reviewed-by: Gary Miguel <garymm@google.com>
diff --git a/infra b/infra
index d8b160c..c418e1f 100644
--- a/infra
+++ b/infra
@@ -33,10 +33,6 @@
              path="chromium/recipes-py"
              remote="https://chromium.googlesource.com/infra/luci/recipes-py"
              gerrithost="https://chromium-review.googlesource.com"/>
-    <project name="chromium/depot_tools"
-             path="chromium/depot_tools"
-             remote="https://chromium.googlesource.com/chromium/tools/depot_tools"
-             gerrithost="https://chromium-review.googlesource.com"/>
   </projects>
   <hooks>
     <hook name="update-prebuilt"