clang9 GitHub Actions builds now failing, and soon to be removed - delete them

[claremacrae]

The weekend build on 2021-05-15 has started failing:

[ 53%] Building CXX object tests/DocTest_Tests/CMakeFiles/DocTest_Tests.dir/ApprovalsTests.cpp.o
In file included from /home/runner/work/ApprovalTests.cpp/ApprovalTests.cpp/tests/UT_Tests/UTApprovalTestTests.cpp:3:
In file included from /home/runner/work/ApprovalTests.cpp/ApprovalTests.cpp/ApprovalTests/ApprovalTests.hpp:86:
In file included from /home/runner/work/ApprovalTests.cpp/ApprovalTests.cpp/ApprovalTests/../ApprovalTests/utilities/DateUtils.h:3:
/usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/chrono:1345:24: error: unknown type name 'strong_ordering'
      friend constexpr strong_ordering
                       ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/chrono:1347:30: error: cannot deduce return type of 'operator<=>' because type 'std::strong_ordering' was not found; include <compare>
      { return unsigned{__x} <=> unsigned{__y}; }
                             ^

Since Clang 9 will be removed on May, 17th anyway, there's no point fixing it - so I'll just deleted the clang9 build GitHub Actions.

I'll also add a separate ticket to add clang12 support, when it's available.

[claremacrae]

This is done