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

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[]