chore: remove unused strong-named signing build infrastructure (#2458)

The buildspec currently contains a prebuild step which fetches a key for
strong-named signing; however, signing has been disabled for at least six months
(see #1770).

Removing the unused code to faciliate cleaning up our infrastructure a bit.

---

By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

buildspec.yaml

@@ -6,9 +6,6 @@ phases:
       # Temporarily - install yarn if it's not there already
       - yarn --version || npm install --global yarn
       - yarn install --frozen-lockfile
-  pre_build:
-    commands:
-      - yarn fetch-dotnet-snk
   build:
     commands:
       - yarn build && yarn test

package.json

@@ -8,7 +8,6 @@
     "lint:fix": "lerna run lint:fix --stream --sort",
     "bump": "bash scripts/bump.sh",
     "dist-clean": "lerna run dist-clean --stream && rm -rf dist",
-    "fetch-dotnet-snk": "bash scripts/fetch-dotnet-snk.sh",
     "package": "bash scripts/package.sh",
     "test": "lerna run test --concurrency=1 --stream",
     "test:integ": "lerna run test:integ --stream",