fix: remove defaultMode for mounted volumes

[corang]

Existing Issue

None currently

Why do we need this PR?

Concourse currently requires to run as root in kubernetes, this boils down to defaultMode being forced on via the helm template, as if you change the uid/gid to anything other than 0:0 (root) it cannot be read.

Changes proposed in this pull request

• Remove defaultMode from the web deployment to allow Concourse to run in a secure way in kubernetes

Contributor Checklist

• Variables are documented in the README.md
• Which branch are you merging into?
  • master is for changes related to the current release of the concourse/concourse:latest image and should be good to publish immediately
  • dev is for changes related to the next release of Concourse (aka unpublished code on master in concourse/concourse)

Reviewer Checklist

This section is intended for the core maintainers only, to track review progress. Please do not
fill out this section.

• Code reviewed
• Topgun tests run
• Back-port if needed
• Is the correct branch targeted? (master or dev)