Remove doctest custom setup (enables cross-compilation)

[georgefst]

Trying to build this library with GHC's Wasm backend (e.g. via nix shell 'gitlab:haskell-wasm/ghc-wasm-meta?host=gitlab.haskell.org') produces the following:

Error: [Cabal-7125]
Failed to build swagger2-2.8.9. The failure occurred during the configure step. The exception was:
  /home/gthomas/.cache/cabal/logs/ghc-9.12.2.20250327/swagger2-2.8.9-26edd33ecfa4f9780f2fed0c672720b9e1d1aab6d590566436f37f650d3eccf7.log: withFile: user error (Error: cabal:
'/nix/store/ni3xwwbkbn9h7vql1ya5svid3jwnrbm7-ghc-wasm/bin/wasm32-wasi-ghc'
exited with an error:
wasm-ld: error: unable to find library -lHSrts-1.0.2_thr
wasm32-wasi-clang: error: linker command failed with exit code 1 (use -v to
see invocation)
wasm32-wasi-ghc-9.12.2.20250327: `wasm32-wasi-clang' failed in phase `Linker'.
(Exit code: 1)
HasCallStack backtrace:
bracket, called at compiler/GHC/Driver/Make.hs:2959:3 in
ghc-9.12.2.20250327-inplace:GHC.Driver.Make

[georgefst]

I should have recognised this error.

It's because of the custom setup, which seemingly only exists for doctest support. There are better ways to run doctests these days, with cabal-doctest being the state-of-the-art as far as I'm aware, at least until "Cabal hooks" have matured.