RFR: 8342997: Remove use of System::getSecurityManager and SecurityManager

[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`).

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Commit messages:
 - 8342994: Remove security manager calls in com.sun.javafx.reflect
 - 8342997: Remove use of System::getSecurityManager and SecurityManager

Changes: https://git.openjdk.org/jfx/pull/1631/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1631&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342997
  Stats: 870 lines in 33 files changed: 10 ins; 829 del; 31 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