Allow full concurrency on Azure Pipelines

Running jobs are cancelled when a new commit is pushed to a PR, so the restriction only ever made sense for Travis.
Plus we have 20 instead of 5 jobs here.

azure-pipelines.yml

@@ -3,29 +3,13 @@ trigger:
   - master
 
 jobs:
-  - job: precheck_concurrent
-    # This job schedules long-running tasks concurrently with the rest of the
-    # build pipeline ensuring that later stages don't block on them
-    # We'll merge it back into the `precheck` stage once Pipelines has caching.
+  - job: precheck
     pool:
       vmImage: 'Ubuntu 16.04'
     strategy:
       matrix:
         check-whole-repo-tests:
           TASK: check-whole-repo-tests
-    steps:
-    - script: sudo apt-get install libreadline-dev libsqlite3-dev
-      displayName: Install apt dependencies
-    - script: ./build.sh check-installed
-      displayName: Install Python
-    - script: ./build.sh
-      displayName: Run build tasks
-
-  - job: precheck
-    pool:
-      vmImage: 'Ubuntu 16.04'
-    strategy:
-      matrix:
         lint:
           TASK: lint
         lint-ruby:
@@ -43,7 +27,6 @@ jobs:
       displayName: Run build tasks
 
   - job: windows
-    dependsOn: precheck
     pool:
       vmImage: 'windows-2019'
     strategy:
@@ -76,7 +59,6 @@ jobs:
       displayName: Run tests
 
   - job: osx
-    dependsOn: precheck
     pool:
       vmImage: 'macOS-10.13'
     strategy:
@@ -95,7 +77,6 @@ jobs:
       displayName: Run tests
 
   - job: linux
-    dependsOn: precheck
     pool:
       vmImage: 'Ubuntu 16.04'
     strategy: