README: Remove references to "make axtls", it's no longer needed.

Since 0be2ea50 axtls is automatically built
as part of the usual "make" build process.

README.md

@@ -76,7 +76,6 @@ To build (see section below for required dependencies):
 
     $ git submodule update --init
     $ cd ports/unix
-    $ make axtls
     $ make
 
 Then to give it a try:

ports/esp8266/README.md

@@ -50,7 +50,6 @@ $ make -C mpy-cross
 Then, to build MicroPython for the ESP8266, just run:
 ```bash
 $ cd ports/esp8266
-$ make axtls
 $ make
 ```
 This will produce binary images in the `build/` subdirectory. If you install