Don't set pid in log context

[hannaeko]

Purpose

Fix regression from #966

Context

-

Changes

• remove line setting pid in context at start of test agent

How to test this PR

• Run test agent and rpcapi
• Start a test
• Check that the pid changes when the test start (59623 and 59634 in the following example)

perl script/zonemaster_backend_testagent --logfile=- --outfile=/tmp/zonemaster_testagent.out --loglevel=debug foreground --logjson | jq -c

{"category":"main","level":"debug","message":"Starting pre-flight check","pid":59623,"timestamp":"2022-05-02T13:21:34Z"}
{"category":"Zonemaster::Backend::Config","level":"notice","message":"Loading config: /home/gbm/workspace/zonemaster/backend_config.ini","pid":59623,"timestamp":"2022-05-02T13:21:34Z"}
{"category":"main","level":"debug","message":"Completed pre-flight check","pid":59623,"timestamp":"2022-05-02T13:21:35Z"}
{"category":"main","level":"notice","message":"Daemon spawned","pid":59623,"timestamp":"2022-05-02T13:21:35Z"}
{"category":"Zonemaster::Backend::DB","level":"notice","message":"Connecting to database 'DBI:Pg:dbname=travis_zonemaster;host=10.10.86.42;port=5432' as user 'travis_zonemaster'","pid":59623,"timestamp":"2022-05-02T13:21:36Z"}
{"category":"main","level":"info","message":"Test found: b6abef8460ef8541","pid":59623,"timestamp":"2022-05-02T13:21:40Z"}
{"category":"main","level":"info","message":"Test starting: b6abef8460ef8541","pid":59634,"timestamp":"2022-05-02T13:21:40Z"}
{"category":"Zonemaster::Backend::DB","level":"notice","message":"Connecting to database 'DBI:Pg:dbname=travis_zonemaster;host=10.10.86.42;port=5432' as user 'travis_zonemaster'","pid":59634,"timestamp":"2022-05-02T13:21:40Z"}
{"category":"main","level":"info","message":"Test completed: b6abef8460ef8541","pid":59634,"timestamp":"2022-05-02T13:21:41Z"}
{"category":"Zonemaster::Backend::Config","level":"notice","message":"Worker process (pid 59634, testid b6abef8460ef8541): Terminated with exit code 0","pid":59623,"timestamp":"2022-05-02T13:21:41Z"}