<Swing Dev> RFR: 8240247: No longer need to wrap files with contentContainer

[Alexey Ivanov]

[JDK-8231144](https://bugs.openjdk.java.net/browse/JDK-8231144) wrapped the contents of plain HTML documents into `<div class="contentContainer">` to apply the styles and add the margins around the content for a consistent look.

This is no longer necessary after [JDK-8239817](https://bugs.openjdk.java.net/browse/JDK-8239817) was fixed.

These documents looks fine without being wrapped. So this fix basically undoes changes under JDK-8231144 and removes the redundant `<div>` wrapper element.

-------------

Commit messages:
 - Remove contentContainer wrapper

Changes: https://git.openjdk.java.net/jdk/pull/2203/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2203&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8240247
  Stats: 45 lines in 15 files changed: 0 ins; 30 del; 15 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2203.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2203/head:pull/2203

PR: https://git.openjdk.java.net/jdk/pull/2203