oss-fuzz.sh: stop downloading the skia seed corpus

oss-fuzz.sh: stop downloading the skia seed corpus

When the fuzz target was integrated, it was added as a stopgap
to get fuzz-json up and running. It served its purpose and can
safely be removed to prevent tools/oss-fuzz.sh from failing with
```
+wget -O /home/travis/build/systemd/systemd/out/fuzz-json_seed_corpus.zip https://storage.googleapis.com/skia-fuzzer/oss-fuzz/skjson_seed_corpus.zip
--2019-09-10 22:40:44--  https://storage.googleapis.com/skia-fuzzer/oss-fuzz/skjson_seed_corpus.zip
Resolving storage.googleapis.com (storage.googleapis.com)... 74.125.70.128, 2607:f8b0:4001:c05::80
Connecting to storage.googleapis.com (storage.googleapis.com)|74.125.70.128|:443... connected.
HTTP request sent, awaiting response... 403 Forbidden
2019-09-10 22:40:44 ERROR 403: Forbidden.
```

Ideally we should put our seed corpus somewhere and download it from there
but I haven't got round to it.