Remove comparator.hh and switch to <=> in a bunch of places

Known behavior changes:

- `MemorySourceAccessor`'s comparison operators no longer forget to
  compare the `SourceAccessor` base class.

Progress on #10832

What remains for that issue is hopefully much easier!

src/libflake/flake/flakeref.cc

@@ -36,11 +36,6 @@ std::ostream & operator << (std::ostream & str, const FlakeRef & flakeRef)
     return str;
 }
 
-bool FlakeRef::operator ==(const FlakeRef & other) const
-{
-    return input == other.input && subdir == other.subdir;
-}
-
 FlakeRef FlakeRef::resolve(ref<Store> store) const
 {
     auto [input2, extraAttrs] = lookupInRegistries(store, input);

src/libflake/flake/flakeref.hh

@@ -46,7 +46,7 @@ struct FlakeRef
      */
     Path subdir;
 
-    bool operator==(const FlakeRef & other) const;
+    bool operator ==(const FlakeRef & other) const = default;
 
     FlakeRef(fetchers::Input && input, const Path & subdir)
         : input(std::move(input)), subdir(subdir)

src/libflake/flake/lockfile.cc

@@ -249,11 +249,6 @@ bool LockFile::operator ==(const LockFile & other) const
     return toJSON().first == other.toJSON().first;
 }
 
-bool LockFile::operator !=(const LockFile & other) const
-{
-    return !(*this == other);
-}
-
 InputPath parseInputPath(std::string_view s)
 {
     InputPath path;

src/libflake/flake/lockfile.hh

@@ -74,9 +74,6 @@ struct LockFile
     std::optional<FlakeRef> isUnlocked() const;
 
     bool operator ==(const LockFile & other) const;
-    // Needed for old gcc versions that don't synthesize it (like gcc 8.2.2
-    // that is still the default on aarch64-linux)
-    bool operator !=(const LockFile & other) const;
 
     std::shared_ptr<Node> findInput(const InputPath & path);