There was no need to use FlyString for error messages, and it just
caused a bunch of churn since these strings typically only existed
during the lifetime of the error.
(cherry picked from commit 175f3febb8037a440d4ead7347d3266ee3d345e1;
minorly amended to fix conflict in WebSocket.cpp due to serenity
not having the adapter class removal in LadybirdBrowser/ladybird#1671)
9b4db4a0e0