x/build/maintner/maintnerd: don't sync GitHub issues once we get a 410 that they're disabled

[bradfitz]

Seen in maintnerd logs:

2019/11/01 01:47:40 sync github golang/arch: Beginning sync.
2019/11/01 01:47:40 sync github golang/arch: 2 missing github issues.
2019/11/01 01:47:40 sync github golang/arch: After page 1: 2 issues, 0 changes, 2 issues in memory
2019/11/01 01:47:40 sync github golang/arch: remaining issues: [1 2]
2019/11/01 01:47:40 sync github golang/arch: getting issue 1 ...
2019/11/01 01:47:40 sync github golang/arch: sync = GET https://api.github.com/repos/golang/arch/issues/1: 410 Issues are disabled for this repo []; sleeping
2019/11/01 01:47:56 sync github golang/debug: Beginning sync.                                                                                                                                   2019/11/01 01:47:57 sync github golang/debug: 1 missing github issues.
2019/11/01 01:47:57 sync github golang/debug: After page 1: 2 issues, 0 changes, 2 issues in memory
2019/11/01 01:47:57 sync github golang/debug: remaining issues: [1]
2019/11/01 01:47:57 sync github golang/debug: getting issue 1 ...
2019/11/01 01:47:57 sync github golang/debug: sync = GET https://api.github.com/repos/golang/debug/issues/1: 410 Issues are disabled for this repo []; sleeping
2019/11/01 01:48:10 sync github golang/review: Beginning sync.
2019/11/01 01:48:10 sync github golang/review: 9 missing github issues.
2019/11/01 01:48:10 sync github golang/review: After page 1: 3 issues, 0 changes, 3 issues in memory
2019/11/01 01:48:10 sync github golang/review: remaining issues: [2 3 4 5 6 7 8 9 10]
2019/11/01 01:48:10 sync github golang/review: getting issue 2 ...
2019/11/01 01:48:10 sync github golang/review: sync = GET https://api.github.com/repos/golang/review/issues/2: 410 Issues are disabled for this repo []; sleeping

Once we see that they're disabled, we should stop polling.