Remove cross-compilation dependencies

It makes the github actions workflow take longer and cross-compiled
artifacts are not currently used (nor do they seem to work).

addons/godot_xterm/native/shell.nix

@@ -1,22 +1,9 @@
-# Use --argstr system i686-linux to build for 32-bit linux.
-{ system ? builtins.currentSystem }:
-with (import <nixpkgs> {
-  inherit system;
-});
+with (import <nixpkgs> {});
 mkShell {
   buildInputs = with pkgs; [
-    binutils.bintools
-    cacert
-    cmake
     git
-    libxkbcommon
-    nix
-    pkg-config
     scons
-  ] ++ lib.optionals (system == builtins.currentSystem) [
-    # Additional dependencies for cross-compiling for Windows and OSX.
-    clang
-    pkgsCross.mingwW64.buildPackages.gcc
-    pkgsCross.mingw32.buildPackages.gcc
+
+    cacert # Required for git clone on GithHub actions runner.
   ];
 }

dockerfiles/archlinux

@@ -1,14 +1,6 @@
 FROM archlinux:20200908
 RUN pacman -Sy --needed --noconfirm \
-    # The following are required to compile for any platform:
     git \
     scons \
-    # The following are required to compile for 64-bit Linux:
-    base-devel \
-    # The following are required to compile for 32-bit Linux:
-    multilib-devel \
-    # The following is required to cross-compile for 32/64-bit Windows:
-    mingw-w64-gcc \
-    # The following is required to cross-compile for MacOS:
-    clang
+    base-devel
 COPY . /src

dockerfiles/ubuntu

@@ -1,16 +1,6 @@
 FROM ubuntu:18.04
 RUN apt-get update && apt-get install -y \
-    # The following are required to compile for any platform:
     git \
-    python3 \
     scons \
-    # The following are required to compile for 64-bit Linux:
-    build-essential \
-    # The following are required to compile for 32-bit Linux:
-    gcc-multilib \
-    g++-multilib \
-    # The following is required to cross-compile for 32/64-bit Windows:
-    g++-mingw-w64-x86-64 \
-    # The following is required to cross-compile for MacOS:
-    clang
+    build-essential
 COPY . /src