Revert "libxbps: implement xbps_file_exec() with posix_spawn()."

This reverts commit 5aa05f4c72.

configure

@@ -305,28 +305,6 @@ if [ -n "$BUILD_PIE" ]; then
 	fi
 fi
 
-#
-# Check for posix_spawn (required).
-#
-func=posix_spawn
-printf "Checking for $func() ... "
-cat <<EOF > _$func.c
-#include <stdio.h>
-#include <spawn.h>
-int main(void) {
-	posix_spawn(NULL, NULL, NULL, NULL, NULL, NULL);
-	return 0;
-}
-EOF
-if $XCC _${func}.c -o _${func} 2>/dev/null; then
-	rm -f _$func.c _$func
-	echo "yes."
-else
-	echo "not found! $func is required by xbps, exiting..."
-	rm -f _$func.c _$func
-	exit 1
-fi
-
 #
 # Check for vasprintf().
 #