fix: Remove content-type header from IDMS GET requests

[peter-volkov]

Detailed description is here https://issuetracker.google.com/issues/328089077

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[parthea]

See https://github.com/googleapis/google-cloud-go/pull/9508#issuecomment-1979100351

[BigTailWolf]

See googleapis/google-cloud-go#9508 (comment)

Hi Anthonios, the golang/oauth2 library is merged. And seems this fix is also good to go.