row: remove InitChecksum for each KV inserted by IMPORT

I noticed `InitChecksum` call show up in the CPU profile of a node from
300 node cluster when IMPORT is running, and I was confused by it. I did
some archaeology, and I think it hasn't been needed for a long time.

For context, prior to 2.0, we effectively required setting the checksum
on the Value since evaluation of CPuts compared RawBytes directly, which
included the checksum comparison. This was an oversight because the
checksum should be optional, and it became such in
33957ae.

However, in order to support IMPORT in mixed-version 1.1-2.0 cluster, in
5ae1bb2 we added the population of the
checksum for all KVs produced for insertion. There is a discussion on
the PR that this is no longer needed as of the optional checksum change
mentioned above.

There is also a very recent discussion about removing this checksum in
non-test code altogether since it probably doesn't give us much since we
do checksum verification at the storage / network level. See #145541.

It should be safe to not populate the checksum for IMPORT as it seems to
be the only place we do so in the SQL land.

Release note: None

Author: yuzefovich

pkg/sql/importer/import_stmt_test.go

@@ -5330,9 +5330,9 @@ func TestImportWorkerFailure(t *testing.T) {
 	)
 }
 
-// TestImportMVCCChecksums verifies that MVCC checksums are correctly
-// computed by issuing a secondary index change that runs a CPut on the
-// index. See #23984.
+// TestImportMVCCChecksums is a regression test for #23984 where we didn't set
+// the MVCC checksums on roachpb.Values produced by the IMPORT when it was
+// required. Doing so is now optional.
 func TestImportMVCCChecksums(t *testing.T) {
 	defer leaktest.AfterTest(t)()
 	defer log.Scope(t).Close(t)

pkg/sql/row/row_converter.go

@@ -349,7 +349,6 @@ func NewDatumRowConverter(
 		db:      db,
 	}
 	c.kvInserter = func(kv roachpb.KeyValue) {
-		kv.Value.InitChecksum(kv.Key)
 		c.KvBatch.KVs = append(c.KvBatch.KVs, kv)
 		c.KvBatch.MemSize += int64(cap(kv.Key) + cap(kv.Value.RawBytes))
 	}