Drop root go.mod dependencies from dependabot (#163)

In order to use a Go library, users must upgrade their dependencies to
be at least as up-to-date as those listed in the library's `go.mod`. As
@abhinav pointed out, using dependabot to then keep our dependencies
up-to-date all the time will cause any users of this library to have to
keep theirs up-to-date all the time as well, when this isn't strictly
necessary due to Go's heavy backwards compatibility culture.

Remove the section referring to the root's `go.mod` so that we can
instead only upgrade dependencies when necessary to avoid too much churn
with users of the library.

.github/dependabot.yaml

@@ -1,10 +1,5 @@
 version: 2
 updates:
-- package-ecosystem: "gomod"
-  directory: "/"
-  schedule:
-    interval: "daily"
-  open-pull-requests-limit: 5
 - package-ecosystem: "gomod"
   directory: "/tools"
   schedule:
@@ -23,4 +18,4 @@ updates:
 - package-ecosystem: "github-actions"
   directory: "/"
   schedule:
-    interval: "weekly"
+    interval: "weekly"