Clean up copy constructor of ScanSpec (facebookincubator#9520)

Summary:

We no longer need to copy and mutate the object, remove the constructor
to make the code cleaner.

Differential Revision: D56267593

velox/dwio/common/ScanSpec.cpp

@@ -19,30 +19,6 @@
 
 namespace facebook::velox::common {
 
-ScanSpec& ScanSpec::operator=(const ScanSpec& other) {
-  if (this != &other) {
-    numReads_ = other.numReads_;
-    subscript_ = other.subscript_;
-    fieldName_ = other.fieldName_;
-    channel_ = other.channel_;
-    constantValue_ = other.constantValue_;
-    projectOut_ = other.projectOut_;
-    extractValues_ = other.extractValues_;
-    makeFlat_ = other.makeFlat_;
-    filter_ = other.filter_;
-    metadataFilters_ = other.metadataFilters_;
-    selectivity_ = other.selectivity_;
-    enableFilterReorder_ = other.enableFilterReorder_;
-    children_ = other.children_;
-    stableChildren_ = other.stableChildren_;
-    childByFieldName_ = other.childByFieldName_;
-    valueHook_ = other.valueHook_;
-    isArrayElementOrMapEntry_ = other.isArrayElementOrMapEntry_;
-    maxArrayElementsCount_ = other.maxArrayElementsCount_;
-  }
-  return *this;
-}
-
 ScanSpec* ScanSpec::getOrCreateChild(const Subfield& subfield) {
   auto container = this;
   auto& path = subfield.path();

velox/dwio/common/ScanSpec.h

@@ -56,12 +56,6 @@ class ScanSpec {
 
   explicit ScanSpec(const std::string& name) : fieldName_(name) {}
 
-  ScanSpec(const ScanSpec& other) {
-    *this = other;
-  }
-
-  ScanSpec& operator=(const ScanSpec&);
-
   // Filter to apply. If 'this' corresponds to a struct/list/map, this
   // can only be isNull or isNotNull, other filtering is given by
   // 'children'.
@@ -358,7 +352,7 @@ class ScanSpec {
   // True if a string dictionary or flat map in this field should be
   // returned as flat.
   bool makeFlat_ = false;
-  std::shared_ptr<common::Filter> filter_;
+  std::unique_ptr<common::Filter> filter_;
 
   // Filters that will be only used for row group filtering based on metadata.
   // The conjunctions among these filters are tracked in MetadataFilter, with