Catch3 is maintained by Progmasoft, but the repository still carried
Catch2's funding links, its maintainer list and GPG keys, a Code of Conduct
contact at a Catch2 maintainer's address, its lists of users, its release
process and release scripts, its AppVeyor configuration and its logos.

- Removed FUNDING.yml, MAINTAINERS.md, appveyor.yml with the AppVeyor and
  OpenCppCoverage helpers, Catch2's user lists, why-catch, release process
  and deprecations pages, its two logos and the major/minor/patch release
  scripts. releaseCommon.py keeps only what the amalgamation reads.
- The Code of Conduct sends reports to support@progmasoft.com.
- contributing.md and release-notes.md are written for Catch3; Catch2's
  release history is linked upstream.
- CMake, pkg-config and Doxygen name Catch3 and its repository; the
  amalgamated files say which Catch3 version and which Catch2 engine they
  come from.

There are two reasons for this:

1) It is highly unlikely that someone has use for this header,
which has no customization points and only provides simplest
possible main, and cannot link the static library which also
provides a default main implementation.
2) It being a header was causing extra complications with
the convenience headers, and our checking script. This would either
require special handling in the checking script, or would break user's
of the main convenience header.

All in all, it is simpler and better in the long term to remove it,
than to fix its problems.

Development build enables warnings and and `Werror` or equivalent.