RFR: 8369531: Wrong generic tooltip used in external class links

[Hannes Wallnöfer]

Please review a change to use the same title attribute (tooltip) in external class links as in internal ones. 

Currently, links to external types and their members always use `class or interface in x.y.z` as tooltip. This is unecessarily generic for type links, because we actually know whether it is a class or an interface, and already have the correct tooltip message. For member links, it is misleading because the link target is not the class or interface.

With this change, external links use the same tooltip as internal ones. Links to top-level types specify whether it is a class or interface. Links to methods use the method signature as tooltip if the link label only contains the method name. For other members, the no tooltip is set. 

I'm also removing the `doclet.Href_Class_Or_Interface_Title` message because it is not used anymore.

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Commit messages:
 - 8369531: Wrong generic tooltip used in external class links

Changes: https://git.openjdk.org/jdk/pull/28491/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28491&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369531
  Stats: 186 lines in 23 files changed: 5 ins; 20 del; 161 mod
  Patch: https://git.openjdk.org/jdk/pull/28491.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28491/head:pull/28491

PR: https://git.openjdk.org/jdk/pull/28491