Remove redundant platform-specific dark theme processors

Display.setDarkThemePreferred(boolean) is a cross-platform SWT API that
signals the OS dark mode preference, and the ThemeEngine already calls it
whenever the CSS theme changes. The platform-specific dark theme processors,
which toggle the same OS preference through internal, reflection-based hooks,
are therefore redundant.

Remove the dark theme processors:
- org.eclipse.e4.ui.swt.gtk (DarkThemeProcessor using OS.setDarkThemePreferred)
- org.eclipse.e4.ui.swt.win32 (DarkThemeProcessor using OS.setTheme)
- org.eclipse.e4.ui.workbench.renderers.swt.cocoa (CocoaDarkThemeProcessor
  using OS.setTheme)

The gtk and win32 fragments only existed to host these processors and are
removed together with their entries in the org.eclipse.e4.rcp feature. The
cocoa fragment is kept because it still provides other processors.

The early display.setDarkThemePreferred(true) call and the manual styling of
the workspace selection dialog in IDEApplication are left untouched, since the
CSS engine is not yet running at that point in the startup sequence.

Fixes #3857

Author: vogella