Remove a couple unnecessary != operators. (#3808)

I believe deprecated by C++20

toolchain/parse/precedence.h

@@ -73,9 +73,6 @@ class PrecedenceGroup {
   friend auto operator==(PrecedenceGroup lhs, PrecedenceGroup rhs) -> bool {
     return lhs.level_ == rhs.level_;
   }
-  friend auto operator!=(PrecedenceGroup lhs, PrecedenceGroup rhs) -> bool {
-    return lhs.level_ != rhs.level_;
-  }
 
   // Compare the precedence levels for two adjacent operators.
   static auto GetPriority(PrecedenceGroup left, PrecedenceGroup right)

toolchain/testing/yaml_test_helpers.h

@@ -63,10 +63,6 @@ struct EmptyComparable {
       -> bool {
     return true;
   }
-  friend auto operator!=(EmptyComparable /*lhs*/, EmptyComparable /*rhs*/)
-      -> bool {
-    return false;
-  }
 };
 
 struct Value;