remove synapse from testing matrix

[emmyoop]

Description & motivation

Removes synapse from testing matrix until we can hook up teh synapse cluster correctly to the blob.

error in CI:

14:14:40  Encountered an error while running operation: Database Error
  ('42000', "[42000] [Microsoft][ODBC Driver 18 for SQL Server][SQL Server]External file access failed due to internal error: 'Parameters provided to connect to the Azure storage account are not valid.' (105019) (SQLExecDirectW)")
dbt_integration_synapse: exit 1 (1413.95 seconds) /project/dbt_packages/dbt_external_tables/integration_tests> dbt run-operation dbt_external_tables.stage_external_sources --vars 'ext_full_refresh: true' --target synapse pid=96
  dbt_integration_synapse: FAIL code 1 (1434.33=setup[0.44]+cmd[1.97,10.97,2.76,4.23,1413.95] seconds)

Checklist

• I have verified that these changes work locally
• I have updated the README.md (if applicable)
• I have added an integration test for my fix/feature (if applicable)