RFR: 8342997: Remove use of System::getSecurityManager and SecurityManager [v4]

[Kevin Rushforth]

> Ths PR removes all remaining calls to `System::getSecurityManager` and the `SecurityManager` class along with the `System.getGecurityManager() != null` code paths.
> 
> As part of doing this, the last uses of `PermissionHelper` and `FXPermissions` were eliminated, so I deleted those two classes. Since `PermissionHelper` was the last remaining use of `AccessControlException`, I am adding JDK-8342998 to this PR.
> 
> In a few places, I noted some "check permissions" methods that are now no-ops that might be candidates for further cleanup (not directly related, so not planned for jfx24).
> 
> Finally, I deleted several unused methods in `MethodUtil` and `ReflectUtil` (some of these were already removed in the corresponding JDK classes, and some of them were unused after the changes to remove the call to `getSecurityManager`).

Kevin Rushforth has updated the pull request incrementally with one additional commit since the last revision:

  remove redundant override of Dragboard::getContentImpl

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1631/files
  - new: https://git.openjdk.org/jfx/pull/1631/files/9dfd64bc..0aac910a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1631&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1631&range=02-03

  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1631.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1631/head:pull/1631

PR: https://git.openjdk.org/jfx/pull/1631