Avoid deleting options.nextFetchPolicy after applying it.

Despite my vague suggestion in the removed comments that deleting
options.nextFetchPolicy somehow helped make the fetchPolicy transition
idempotent (a concern that only matters when nextFetchPolicy is a
function), leaving options.nextFetchPolicy intact should also be
safe/idempotent in virtually all cases, including every case where
nextFetchPolicy is just a string, rather than a function.

More importantly, leaving options.nextFetchPolicy intact allows it to be
applied more than once, which seems to fix issue #6839.

src/__tests__/client.ts

@@ -3319,7 +3319,6 @@ describe('@connection', () => {
               // again to perform an additional transition.
               this.nextFetchPolicy = fetchPolicy => {
                 ++nextFetchPolicyCallCount;
-                expect(fetchPolicy).toBe("cache-and-network");
                 return "cache-first";
               };
               return "cache-and-network";
@@ -3372,9 +3371,8 @@ describe('@connection', () => {
           client.cache.evict({ fieldName: "count" });
         } else if (handleCount === 6) {
           expect(result.data).toEqual({ count: 2 });
-          expect(nextFetchPolicyCallCount).toBe(3);
+          expect(nextFetchPolicyCallCount).toBe(4);
           expect(obs.options.fetchPolicy).toBe("cache-first");
-          expect(obs.options.nextFetchPolicy).toBeUndefined();
           setTimeout(resolve, 50);
         } else {
           reject("too many results");

src/core/ObservableQuery.ts

@@ -732,11 +732,6 @@ export function applyNextFetchPolicy<TData, TVars>(
   } = options;
 
   if (nextFetchPolicy) {
-    // The options.nextFetchPolicy transition should happen only once, but it
-    // should also be possible (though uncommon) for a nextFetchPolicy function
-    // to set options.nextFetchPolicy to perform an additional transition.
-    options.nextFetchPolicy = void 0;
-
     // When someone chooses "cache-and-network" or "network-only" as their
     // initial FetchPolicy, they often do not want future cache updates to
     // trigger unconditional network requests, which is what repeatedly