Remove non-existent import for gemma (tenstorrent#28474)

### Ticket
Gemma tests are failing on import of
`models/tt_transformers/test_accuracy.py` which was removed in a recent
PR. See:
https://github.com/tenstorrent/tt-metal/actions/runs/17692049855/job/50290389980#step:10:360

This PR removes the import - the function was already copied across.

Author: handrewsTT

models/demos/gemma3/demo/text_demo.py

@@ -17,7 +17,6 @@
 from models.demos.gemma3.tt.model_config import determine_device_name, parse_decoder_json
 from models.demos.utils.llm_demo_utils import create_benchmark_data, verify_perf
 from models.perf.benchmarking_utils import BenchmarkProfiler
-from models.tt_transformers.tests.test_accuracy import get_accuracy_thresholds
 from models.tt_transformers.tt.common import PagedAttentionConfig, preprocess_inputs_prefill, sample_host
 from models.tt_transformers.tt.generator import Generator, SamplingParams, create_submeshes
 from models.tt_transformers.tt.model_config import DecodersPrecision