Drop single user support on Che Server

[xbaran4]

What does this PR do?

Removes checking for single-user mode
Removes classes and modules used only in single-user mode
Removes H2 dependecies

Screenshot/screencast of this PR

What issues does this PR fix or reference?

Implements eclipse-che/che#20314

How to test this PR?

Che server would go into single-user mode when installed with helm.
Installation with helm is now deprecated eclipse-che/che#20183
With this PR, che server now goes straight into multiuser mode and is therefore dependent on postgreSQL and id provider and will crash on helm installation.
So, to test the multiuser mode I deployed che on openshift with patch

spec:
  server:
    cheImage: quay.io/pbaran/che-server
    cheImagePullPolicy: Always
    cheImageTag: singleuser

and created a workspace.

PR Checklist

As the author of this Pull Request I made sure that:

• The Eclipse Contributor Agreement is valid
• Code produced is complete
• Code builds without errors
• Tests are covering the bugfix
• The repository devfile is up to date and works
• Sections What issues does this PR fix or reference and How to test this PR completed
• Relevant user documentation updated
• Relevant contributing documentation updated
• CI/CD changes implemented, documented and communicated

Reviewers

Reviewers, please comment how you tested the PR when approving it.

[che-bot]

Can one of the admins verify this patch?

[metlos]

Oh, deleting that much code must have felt great :) Good job!