RFR: 8342993: Remove uses of AccessController and AccessControlContext from JavaFX

[Kevin Rushforth]

This PR removes all remaining uses of `AccessController` and `AccessControlContext`, which represent the last remaining uses of the terminally deprecated security APIs except for those in the `/ios/` or `/android/` directories.

With the removal of doPrivileged and the `if (System.getSecurityManager() != null)` code paths, the ACC is no longer used, so can be completely eliminated. Along with this, I removed all unused imports of security-related APIs and all related `@SuppressWarnings("removal") annotations.

### Notes to reviewers

* Most of the changes were straight-forward removals of methods and fields to save, retrieve and pass around the `AccessControlContext`.
* The Toolkit class stores a collection of listeners in a `WeakHashMap` with the listener as the key (thus weakly held) and the ACC as the value. We no longer need or want the ACC, but I kept the use of `WeakHashMap` and changed the value type to `Object`, storing a singleton dummy object as the value for each entry. This minimizes the changes, while preserving the behavior of reclaiming the entries when they are garbage collected.

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Commit messages:
 - 8342993: Remove uses of AccessController and AccessControlContext from JavaFX

Changes: https://git.openjdk.org/jfx/pull/1638/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1638&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342993
  Stats: 324 lines in 30 files changed: 7 ins; 260 del; 57 mod
  Patch: https://git.openjdk.org/jfx/pull/1638.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1638/head:pull/1638

PR: https://git.openjdk.org/jfx/pull/1638