Only register if instrument not already registered

Rather than having a configuration option for registration,
check to see if the registry already knows about that instrument.

python/lsst/obs/base/gen2to3/convertRepo.py

@@ -156,13 +156,6 @@ class ConvertRepoConfig(Config):
         itemtype=str,
         default={}
     )
-    doRegisterInstrument = Field(
-        "If True (default), add dimension records for the Instrument and its "
-        "filters and detectors to the registry instead of assuming they are "
-        "already present.",
-        dtype=bool,
-        default=True,
-    )
     doWriteCuratedCalibrations = Field(
         "If True (default), ingest human-curated calibrations directly via "
         "the Instrument interface.  Note that these calibrations are never "
@@ -463,13 +456,13 @@ def run(self, root: str, collections: List[str], *,
         # runs/collections, and transactions are managed at the Butler level.
         # DM-21246 should let us fix this, assuming we actually want to keep
         # the transaction open that long.
-        if self.config.doRegisterInstrument:
-            # Allow registration to fail on the assumption that this means
-            # we are reusing a butler
-            try:
-                self.instrument.register(self.registry)
-            except Exception:
-                pass
+        # Only register an instrument if it has not been previously registered
+        try:
+            # Need the list to force the query to happen
+            list(self.registry.queryDimensions("instrument",
+                                               instrument=self.instrument.getName()))
+        except LookupError:
+            self.instrument.register(self.registry)
 
         # Make and prep converters for all Gen2 repos.  This should not modify
         # the Registry database or filesystem at all, though it may query it.