scripted-diff: Drop Darwin version for better maintainability

-BEGIN VERIFY SCRIPT-
sed -i 's/darwin19/darwin/g' $(git grep --files-with-matches 'darwin19')
-END VERIFY SCRIPT-

contrib/guix/guix-build

@@ -76,7 +76,7 @@ mkdir -p "$VERSION_BASE"
 # Default to building for all supported HOSTs (overridable by environment)
 export HOSTS="${HOSTS:-x86_64-linux-gnu arm-linux-gnueabihf aarch64-linux-gnu riscv64-linux-gnu powerpc64-linux-gnu powerpc64le-linux-gnu
                        x86_64-w64-mingw32
-                       x86_64-apple-darwin19}"
+                       x86_64-apple-darwin}"
 
 # Usage: distsrc_for_host HOST
 #