opt: remove a stale comment above optbuilder.buildScan

Removed a comment that references a function parameter that no longer
exists.

Release note: None

pkg/sql/opt/optbuilder/select.go

@@ -523,18 +523,6 @@ func errorOnInvalidMultiregionDB(
 // be in the list (in practice, this coincides with all "ordinary" table columns
 // being in the list).
 //
-// If scanMutationCols is true, then include columns being added or dropped from
-// the table. These are currently required by the execution engine as "fetch
-// columns", when performing mutation DML statements (INSERT, UPDATE, UPSERT,
-// DELETE).
-//
-// NOTE: Callers must take care that mutation columns (columns that are being
-//
-//	added or dropped from the table) are only used when performing mutation
-//	DML statements (INSERT, UPDATE, UPSERT, DELETE). They cannot be used in
-//	any other way because they may not have been initialized yet by the
-//	backfiller!
-//
 // See Builder.buildStmt for a description of the remaining input and return
 // values.
 func (b *Builder) buildScan(