Remove (unneeded?) parts of the pipeline

.azure/azure-pipelines.yml

@@ -1,5 +1,4 @@
 variables:
-  protobuf-version: 3.7.1
   lkg-rust-nightly: "2019-06-16"
 
 trigger:
@@ -10,18 +9,12 @@ strategy:
     linux-nightly:
       image: ubuntu-16.04
       toolchain: nightly-$(lkg-rust-nightly)
-      features: unstable
-      protobuf-platform: linux-x86_64
     macos-nightly:
       image: macos-10.13
       toolchain: nightly-$(lkg-rust-nightly)
-      features: unstable
-      protobuf-platform: osx-x86_64
     windows-nightly:
       image: vs2017-win2016
       toolchain: nightly-$(lkg-rust-nightly)
-      features: unstable
-      protobuf-platform: win64
 
 pool:
   vmImage: $(image)
@@ -39,4 +32,4 @@ steps:
   - bash: cargo build
     displayName: Build source
   - bash: cargo test
-    displayName: Run tests
+    displayName: Run tests