Remove `SYNC_QUOTA` from env template

[mxab]

Is your feature request related to a problem? Please describe.
When reading the configuration parameters for core to understand what can be configured I came across SYNC_QUOTA which doesn't seem to be used anywhere and is not documented anymore.

I think the variable was used in a feature that has been removed

Describe the solution you'd like
Remove the SYNC_QUOTA entry from make/photon/prepare/templates/core/env.jinja

Additional context

Reference Search for SYNC_QUOTA in harbor

[bupd]

We can get rid of this

https://github.com/search?q=org%3Agoharbor%20sync_quota&type=code