Bug 1647310 - Stop storing charset on cache entries. r=necko-reviewers,dragana

Storing the charset on cache entries makes the code path uselessly different
when loading from cache relative to uncached loads. Also, for future
telemetry purposes, caching the charset obscures its original source.

Differential Revision: https://phabricator.services.mozilla.com/D101570

docshell/base/nsDocShell.cpp

@@ -1472,7 +1472,6 @@ nsDocShell::GatherCharsetMenuTelemetry() {
       break;
     case kCharsetFromFallback:
     case kCharsetFromDocTypeDefault:
-    case kCharsetFromCache:
     case kCharsetFromParentFrame:
       // Changing charset on an unlabeled doc.
       if (isFileURL) {