Revert "build: add full Python 3 tests to Travis CI"

Reverted for breaking the Travis CI builds with the following error:

    $ cp ${PYTHON2_CACHE}/node out/Release/node

    cp: cannot stat '/home/travis/.ccache/py2.7.15/node': No such file
    or directory

    The command "cp ${PYTHON2_CACHE}/node out/Release/node" failed and
    exited with 1 during .

This reverts commit c602b0a4.

Refs: https://github.com/nodejs/node/pull/29360

PR-URL: https://github.com/nodejs/node/pull/29406


Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>