[Haskell][Servant] Remove switch that is no longer needed due to dependency change

[Drezil]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Implemented fix for #1849

[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[Drezil]

Oh. Forgot to add that email-address to my profile. Fixed now. Thanks for the notice.

[wing328]

cc @jonschoning @algas

[wing328]

Build passed locally:

➜  haskell-servant git:(Drezil-master) stack test
openapi-petstore-0.1.0.0: configure (lib)
Configuring openapi-petstore-0.1.0.0...
clang: warning: argument unused during compilation: '-nopie' [-Wunused-command-line-argument]
openapi-petstore-0.1.0.0: build (lib)
Preprocessing library for openapi-petstore-0.1.0.0..
Building library for openapi-petstore-0.1.0.0..
[1 of 2] Compiling OpenAPIPetstore.Types ( lib/OpenAPIPetstore/Types.hs, .stack-work/dist/x86_64-osx/Cabal-2.2.0.1/build/OpenAPIPetstore/Types.o )
[2 of 2] Compiling OpenAPIPetstore.API ( lib/OpenAPIPetstore/API.hs, .stack-work/dist/x86_64-osx/Cabal-2.2.0.1/build/OpenAPIPetstore/API.o )
openapi-petstore-0.1.0.0: copy/register
Installing library in /Users/williamcheng/Code/openapi-generator/samples/server/petstore/haskell-servant/.stack-work/install/x86_64-osx/lts-12.18/8.4.4/lib/x86_64-osx-ghc-8.4.4/openapi-petstore-0.1.0.0-HC9DnrEnAtD3ME0hmTe0rs
Registering library for openapi-petstore-0.1.0.0..