Remove redundant builds and confusing indirection (#300)

Jenkinsfile

@@ -20,8 +20,11 @@ node('linux-jammy-unprovisioned') {
           stage('setup') {
             sh './scripts/continuous_integration/jenkins/setup'
           }
-          stage('build and test') {
-            sh './scripts/continuous_integration/jenkins/build_test'
+          stage('bazel_external build and test') {
+            sh './scripts/continuous_integration/common/drake_bazel_external'
+          }
+          stage('cmake_external build and test') {
+            sh './scripts/continuous_integration/common/drake_cmake_external'
           }
         }
       } catch (e) {

README.md

@@ -16,16 +16,16 @@ own project:
 Scripts are provided for various CI instances in `scripts/continuous_integration`. The intended purpose of each is described below:
 
 * `github_actions`:  exemplifies how to put a project depending on a Drake installation on GitHub Actions
-* `jenkins` : provides complete coverage of additional example projects
+* `jenkins` : provides complete coverage of additional external example projects
 
 | **Subproject** | **GitHub Actions** | **Jenkins** |
 |:---:|:---:|:---:|
 | `drake_ament_cmake_installed` | o | - |
 | `drake_bazel_external` | - | o |
-| `drake_bazel_download` | o | o |
-| `drake_catkin_installed` | o | o |
+| `drake_bazel_download` | o | - |
+| `drake_catkin_installed` | o | - |
 | `drake_cmake_external` | - | o |
-| `drake_cmake_installed` | o | o |
+| `drake_cmake_installed` | o | - |
 | `drake_cmake_installed_apt` | o | - |
 || ![GitHub Actions](https://img.shields.io/github/actions/workflow/status/RobotLocomotion/drake-external-examples/ci.yml?branch=main) | [![Jenkins](https://img.shields.io/jenkins/build.svg?jobUrl=https://drake-jenkins.csail.mit.edu/job/RobotLocomotion/job/drake-external-examples/job/main)](https://drake-jenkins.csail.mit.edu/job/RobotLocomotion/job/drake-external-examples/) |