Remove Gateway documentation (moved to downstream Red Hat OpenShift Dev Spaces) (#2995) (#2997)

The JetBrains Gateway integration is specific to Red Hat OpenShift Dev Spaces
and requires the Gateway provider which is not available in upstream Eclipse Che.

Removed:
- modules/end-user-guide/pages/idea-ultimate.adoc
- modules/end-user-guide/images/editor-idea-iu.png
- modules/end-user-guide/images/gateway-connect.png
- modules/end-user-guide/images/gateway-connecting.png
- modules/end-user-guide/images/gateway-select-ws.png
- modules/end-user-guide/images/gateway.png
- modules/end-user-guide/images/open-gateway-prompt.png
- Navigation reference in modules/end-user-guide/nav.adoc

Note: github-action-*.png images are kept as they are used by the
'Try in Web IDE' GitHub Action feature (not Gateway-specific).

Author: gtrivedi88

modules/end-user-guide/images/editor-idea-iu.png

@@ -31,7 +31,6 @@
 * xref:ides-in-workspaces.adoc[]
 ** xref:microsoft-visual-studio-code-open-source-ide.adoc[]
 ** xref:connect-visual-studio-code-to-che-workspace.adoc[]
-** xref:idea-ultimate.adoc[]
 ** xref:defining-a-common-ide.adoc[]
 * xref:using-credentials-and-configurations-in-workspaces.adoc[]
 ** xref:mounting-secrets.adoc[]