Remove CodeCov support

[php-coder]

The codecov bot doesn't do anything useful to us. Also, we don't care much about code coverage and numbers. JaCoCo is enough. Let's decommission CodeCov.

• update NEWS.txt
• update README.md (remove a badge)
• remove publish_code_coverage.sh + update .travis.yml
• GitHub: decide whether we need to revoke an access or not
• GitHub: remove a webhook
• (probably) disable integration on CodeCov site

[php-coder]

GitHub: decide whether we need to revoke an access or not

Let's keep it as it can't be undone.

[php-coder]

(probably) disable integration on CodeCov site

A whole account has been deleted.

[php-coder]

Done.