[common] Remove no-longer-needed DEP

BUG=skia:

Review URL: https://codereview.chromium.org/2436403002 .
diff --git a/DEPS b/DEPS
index c866727..096f7c1 100644
--- a/DEPS
+++ b/DEPS
@@ -3,5 +3,4 @@
 # Dependencies on outside packages.
 #
 deps = {
-  "third_party/externals/boto" : "https://github.com/boto/boto.git@410909e993746c1b7a2604721d82afc0d2778a8a",
 }