fix: remove static AUTH_PROVIDER in istio.env to allow operator to set it at runtime

[dhirajsb]

Description

Removed default value of AUTH_PROVIDER in istio.env

How Has This Been Tested?

Tested by creating sample MR using the updated istio.env locally

Merge criteria:

• The commits and have meaningful messages; the author will squash them after approval or will ask to merge with squash.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

[rareddy]

Can you describe why we need this, do we need to push this downstream?

[dhirajsb]

@rareddy since it is in the samples we don't need to push it to downstream. The auth provider name is different between odh and rhoai. But the name is actually passed along in an env variable from odh/rhods operator to MR operator.
So, leaving this property blank allows MR operator to automatically use the environment default.

[dhirajsb]

@tonyxrmdavidson has already fixed this in QE ods-ci tests.