Remove the manylinux workflow

This has been superseded by the `wheel` workflow. Although Jim was
[hesitant for it to be removed][0], it's out-of-date and broken and we
just need to get wheel builds working correctly at this point.

[0]: xmlsec#334

Author: ngnpope

.github/workflows/manylinux.yml

@@ -6,8 +6,6 @@ python-xmlsec
 .. image:: https://results.pre-commit.ci/badge/github/xmlsec/python-xmlsec/master.svg
    :target: https://results.pre-commit.ci/latest/github/xmlsec/python-xmlsec/master
    :alt: pre-commit.ci status
-.. image:: https://github.com/mehcode/python-xmlsec/actions/workflows/manylinux.yml/badge.svg
-   :target: https://github.com/mehcode/python-xmlsec/actions/workflows/manylinux.yml
 .. image:: https://github.com/mehcode/python-xmlsec/actions/workflows/macosx.yml/badge.svg
    :target: https://github.com/mehcode/python-xmlsec/actions/workflows/macosx.yml
 .. image:: https://github.com/mehcode/python-xmlsec/actions/workflows/linuxbrew.yml/badge.svg