gopls/internal/telemetry/cmd/stacks: delete

It moved to x/telemetry/cmd/stacks in CL 678275.

Change-Id: I8e91068dc8227d37b34720ff3f2e8311f899bee6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/678295
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan