Revert "travis: Build esp8266 firmware as part of Travis CI."

This reverts commit cc12f75. This
implementation is mediocre comparing to Pycopy's, e.g. uses some
random toolchain blobs.

Change-Id: If0d0891d1cce69c590228183c6aa91acea034699

.travis.yml

@@ -206,18 +206,6 @@ jobs:
         - make ${MAKEOPTS} -C mpy-cross
         - make ${MAKEOPTS} -C ports/esp32 ESPIDF=$(pwd)/esp-idf
 
-    # esp8266 port
-    - stage: test
-      env: NAME="esp8266 port build"
-      install:
-        - wget https://github.com/jepler/esp-open-sdk/releases/download/2018-06-10/xtensa-lx106-elf-standalone.tar.gz
-        - zcat xtensa-lx106-elf-standalone.tar.gz | tar x
-        - export PATH=$(pwd)/xtensa-lx106-elf/bin:$PATH
-      script:
-        - git submodule update --init lib/axtls lib/berkeley-db-1.xx
-        - make ${MAKEOPTS} -C mpy-cross
-        - make ${MAKEOPTS} -C ports/esp8266
-
     # nrf port
     - stage: test
       env: NAME="nrf port build"