Removed branch 7.3 from CI testing

.ci/jobs/elastic+elasticsearch-js+7.3.yml

@@ -1,14 +0,0 @@
----
-- job:
-    name: elastic+elasticsearch-js+7.3
-    display-name: 'elastic / elasticsearch-js # 7.3'
-    description: Testing the elasticsearch-js 7.3 branch.
-    parameters:
-    - string:
-        name: branch_specifier
-        default: refs/heads/7.3
-        description: the Git branch specifier to build (<branchName>, <tagName>,
-          <commitId>, etc.)
-    triggers:
-    - github
-    - timed: '@weekly'