http: switch on string values

Long ago, V8 was much faster switching on string lengths than values.
That is no longer the case, so we can simplify a couple of methods.

PR-URL: https://github.com/nodejs/node/pull/18351


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Kyle Farnung <kfarnung@microsoft.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>