Remove Appspot-facing proxy-go instances

Currently we run 7 round-the-clock instances of proxy-go:

1. snowflake-proxy-restartless
2. snowflake-proxy-17h
3. snowflake-proxy-23h
4. snowflake-proxy-29h
5. snowflake-proxy-standalone-17h
6. snowflake-proxy-standalone-23h
7. snowflake-proxy-standalone-29h

snowflake-proxy-restartless is an experiment for legacy/trac#25688 (moved), so put it aside. Of the remaining 6, the only difference is what broker they correspond with:

• 3 for !https://snowflake-reg.appspot.com/ (the old App Engine–bound broker)
• 3 for !https://snowflake-broker.bamsoftware.com/ (the current standalone broker)

We fully switched away from the App Engine broker [[comment:9:ticket:22874|10 months ago]]. Nothing in the source code refers to snowflake-reg.appspot.com any longer. So I propose we stop running the 3 instances that use the old App Engine broker:

• snowflake-proxy-17h
• snowflake-proxy-23h
• snowflake-proxy-29h