Remove claims that "constexpr functions are pure" (#1307)

F.4 and F.8 made this erroneous claim as parts of notes which are
entirely removed.

CppCoreGuidelines.md

@@ -2601,25 +2601,6 @@ it just guarantees that the function can be evaluated at compile time for consta
 
 ##### Note
 
-`constexpr` functions are pure: they can have no side effects.
-
-    int dcount = 0;
-    constexpr int double(int v)
-    {
-        ++dcount;   // error: attempted side effect from constexpr function
-        return v + v;
-    }
-
-This is usually a very good thing.
-
-When given a non-constant argument, a `constexpr` function can throw.
-If you consider exiting by throwing a side effect, a `constexpr` function isn't completely pure;
-if not, this is not an issue.
-??? A question for the committee: can a constructor for an exception thrown by a `constexpr` function modify state?
-"No" would be a nice answer that matches most practice.
-
-##### Note
-
 Don't try to make all functions `constexpr`.
 Most computation is best done at run time.
 
@@ -2795,16 +2776,6 @@ Pure functions are easier to reason about, sometimes easier to optimize (and eve
     template<class T>
     auto square(T t) { return t * t; }
 
-##### Note
-
-`constexpr` functions are pure.
-
-When given a non-constant argument, a `constexpr` function can throw.
-If you consider exiting by throwing a side effect, a `constexpr` function isn't completely pure;
-if not, this is not an issue.
-??? A question for the committee: can a constructor for an exception thrown by a `constexpr` function modify state?
-"No" would be a nice answer that matches most practice.
-
 ##### Enforcement
 
 Not possible.