cmd/getgo: delete package

The last version v0.1.0-deprecated was published.

Fixes golang/go#60951

Change-Id: Ib178384ba3a4010edd509825f8a9106853ccf703
Reviewed-on: https://go-review.googlesource.com/c/tools/+/564657
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Auto-Submit: Hyang-Ah Hana Kim <[email protected]>

Author: hyangah