Remove RAB Env Variable GOOGLE_AUTH_TRUST_BOUNDARY_ENABLE_EXPERIMENT

[vverman]

Currently,
These test cases are failing because of removing Env variable gating for RAB refresh

1 -> https://github.com/googleapis/google-auth-library-java/actions/runs/22943747971/job/66590971922
2 -> https://github.com/googleapis/google-auth-library-java/actions/runs/22992053949/job/66755089161
3 -> https://github.com/googleapis/google-auth-library-java/actions/runs/22872508322/job/66355652494

For this, GOOGLE_AUTH_TRUST_BOUNDARY_ENABLE_EXPERIMENT is left in the RAB PR which will be removed before the RAB release.