Remove copy_bool workaround for gcc 4.3

Reference issue

What does this implement/fix?

This is a split-off from MR !881 .

The testsuite uses a workaround for gcc 4.3, a noughties-era compiler which implements something called C++0x as its highest version of C++. In other words, this compiler can no longer be used to compile Eigen. Torn between turning this workaround into a constexpr and outright removing it, I opted for the latter.

Additional information

I ran the first few hundred testsuite checks on my PC and found no regressions.