RFR: 2244: SecurityLevel in JiraHost is useless

[Zhao Song]

SecurityLevel in JiraHost is rarely used and sometimes it will conflict with the security configuration of issueNotifier. So there is no point to keep it. Also, I removed the support of `/issue create` command in this patch as ErikJ suggested. 

There is one useful usage of `securityLevel`, it's in `JiraIssue#addWebLink`, skara bot uses the security to determine if the link should be posted as a comment. But I think it can be replaced by checking the visibilityRole of the jiraHost.

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Commit messages:
 - SKARA-2244

Changes: https://git.openjdk.org/skara/pull/1642/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1642&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2244
  Stats: 234 lines in 7 files changed: 0 ins; 221 del; 13 mod
  Patch: https://git.openjdk.org/skara/pull/1642.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1642/head:pull/1642

PR: https://git.openjdk.org/skara/pull/1642