RFR: JDK-8263104: fix warnings for empty paragraphs

[Jonathan Gibbons]

Please review some simple cleanup for empty `<p>` tags.

Two of the tags are completely redundant, and simply deleted.

The other three, in _package.html_ files are generally undesirable. Although the presumed intent of the `id` declaration is to label the `@see` info, proximity in the source code does not ensure proximity in the generated code. The actual result is an empty paragraph at the end of the enclosing generated `<div>`, and before the generated output for the `@since` tag.

The better solution is to move the `id` declaration into the `@see <a href...` and then delete the empty `<p>`.

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Commit messages:
 - JDK-8263104: fix warnings for empty paragraphs

Changes: https://git.openjdk.java.net/jdk/pull/2850/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2850&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263104
  Stats: 8 lines in 5 files changed: 0 ins; 4 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2850.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2850/head:pull/2850

PR: https://git.openjdk.java.net/jdk/pull/2850