Remove references to GTK+ and WebKitGTK+ from README

https://bugs.webkit.org/show_bug.cgi?id=264161

Reviewed by Carlos Garcia Campos.

GTK+ was renamed to GTK in GTK 4, and we renamed WebKitGTK+ to WebKitGTK
correspondingly. Don't put punctuation in names. Just don't. Looking at
you, Pop!_OS.

* ReadMe.md:

Canonical link: https://commits.webkit.org/270183@main

ReadMe.md

@@ -68,7 +68,7 @@ Tools/Scripts/build-webkit --debug --<platform>-device
 
 where `platform` is `ios`, `tvos` or `watchos`.
 
-### Building the GTK+ Port
+### Building the GTK Port
 
 For production builds:
 
@@ -86,7 +86,7 @@ Tools/Scripts/update-webkitgtk-libs
 Tools/Scripts/build-webkit --gtk --debug
 ```
 
-For more information on building WebKitGTK+, see the [wiki page](https://trac.webkit.org/wiki/BuildingGtk).
+For more information on building WebKitGTK, see the [wiki page](https://trac.webkit.org/wiki/BuildingGtk).
 
 ### Building the WPE Port