Deduplicate CreateCertificateFromCSR fixtures

[m5]

When this repository is cloned on OSX, CreateCertificateFromCSR.yaml is overwritten by CreateCertificateFromCsr.yaml.

Nix uses a hash checksum to verify the repository, and with those files missing, a hash generated on OSX won't match one generated on linux.

NixOS/cabal2nix#382

The conflicting files are all empty, and CreateCertificateFromCSR seems to be the preferred variant.

[MaxGabriel]

For users of git 2.21, this PR will also fix the following warning when cloning the repo to a case-insensitive file system:

✔︎ /p/tmp> git clone https://github.com/brendanhay/amazonka.git          16:46:10
Cloning into 'amazonka'...
remote: Enumerating objects: 5, done.
remote: Counting objects: 100% (5/5), done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 283101 (delta 4), reused 2 (delta 2), pack-reused 283096
Receiving objects: 100% (283101/283101), 162.65 MiB | 1.10 MiB/s, done.
Resolving deltas: 100% (214271/214271), done.
Checking out files: 100% (15636/15636), done.
warning: the following paths have collided (e.g. case-sensitive paths
on a case-insensitive filesystem) and only one from the same
colliding group is in the working tree:

  'amazonka-iot/fixture/CreateCertificateFromCSR.yaml'
  'amazonka-iot/fixture/CreateCertificateFromCsr.yaml'
  'amazonka-iot/fixture/CreateCertificateFromCSRResponse.proto'
  'amazonka-iot/fixture/CreateCertificateFromCsrResponse.proto'

[purefn]

@brendanhay - can we get this merged? It's making it impossible to share nix expressions for amazonka between Linux and macOS users.