Remove unused 32-bit libXext dependency

When the compilation instructions were first written [1], it made sense
to tell users to install the 32-bit version of libXext. At the time,
compiling Descent 3 on Linux required running a pre-compiled
hogUtils-i686 binary, and that binary depended on libXext.

As of 1765d07 (Remove now unused hogUtils-i686, 2024-04-21), the
hogUtils binary no longer exists, so there’s no need to install a 32-bit
version of libXext.

[1]: f929310 (Update README.md, 2024-04-20)

README.md

@@ -50,18 +50,16 @@ cmake --build --preset mac --config [Debug|Release]
 
 #### Building - Linux (Ubuntu)
 ```sh
-sudo dpkg --add-architecture i386
 sudo apt update
-sudo apt install -y --no-install-recommends ninja-build cmake g++ libsdl1.2-dev libsdl-image1.2-dev libncurses-dev libxext6:i386 zlib1g-dev libspdlog-dev
+sudo apt install -y --no-install-recommends ninja-build cmake g++ libsdl1.2-dev libsdl-image1.2-dev libncurses-dev zlib1g-dev libspdlog-dev
 cmake --preset linux -D LOGGER=[ON|OFF]
 cmake --build --preset linux --config [Debug|Release]
 ```
 
 #### Building - Linux (Fedora)
 ```sh
 sudo dnf update --refresh
-sudo dnf install -y ninja-build cmake gcc-c++ SDL-devel SDL_image-devel ncurses-devel libXext.x86_64 zlib-devel spdlog-devel
-sudo dnf install -y libXext.i686
+sudo dnf install -y ninja-build cmake gcc-c++ SDL-devel SDL_image-devel ncurses-devel zlib-devel spdlog-devel
 cmake --preset linux -D LOGGER=[ON|OFF]
 cmake --build --preset linux --config [Debug|Release]
 ```