servo: Merge #16468 - Remove windows-gnu build code (from aneeshusa:remove-windows-gnu); r=metajack

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This is a follow-up to servo/saltfs#620.

There are three other places `windows-gnu` is mentioned, I'm not sure if they can also be removed or not:
- `nightly-x86_64-pc-windows-gnu` is listed as a target in the `appveyor.yml` build matrix
- 2 uses inside `host_platform` in `python/servo/util.py`
- The `windows_gnu` bootstrapper in `python/servo/bootstrap.py`

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because they remove unused code

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Source-Repo: https://github.com/servo/servo
Source-Revision: 7ae9c96467b1a80f104aa5346c9fd11522490541

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 8e1b943fe6b0dc1bed06d58fcb3aa6d67d51271a

servo/etc/ci/buildbot_steps.yml

@@ -100,21 +100,11 @@ arm64:
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
 
-windows-gnu-dev:
-  - ./mach build --dev
-  - ./mach test-unit
-  - ./mach build-geckolib
-
 windows-msvc-dev:
   - mach.bat build --dev
   - mach.bat test-unit
   - mach.bat build-geckolib
 
-windows-gnu-nightly:
-  - ./mach build --release
-  - ./mach package --release
-  - ./etc/ci/upload_nightly.sh windows-gnu
-
 windows-msvc-nightly:
   - mach.bat build --release
   - mach.bat package --release

servo/etc/ci/upload_nightly.sh

@@ -12,7 +12,7 @@ shopt -s failglob
 
 usage() {
     printf \
-        "usage: %s android|linux|mac|macbrew|windows-gnu|windows-msvc\n" \
+        "usage: %s android|linux|mac|macbrew|windows-msvc\n" \
         "${0}"
 }
 
@@ -89,8 +89,7 @@ main() {
     elif [[ "${platform}" == "macbrew" ]]; then
         extension=tar.gz
         package=target/release/brew/*."${extension}"
-    elif [[ "${platform}" == "windows-gnu" ||
+    elif [[ "${platform}" == "windows-msvc" ]]; then
-            "${platform}" == "windows-msvc" ]]; then
         extension=msi
         package=target/release/msi/*.msi
     else