Remove MXNET_ENGINE_TYPE settings from Travis CI config

Since `Build.mak` now takes care of running tests with all engines, we
can greatly simplify the number and configuration of Travis CI jobs.
This patch reduces the setup to cover development and production builds
for D1 and D2 respectively.

.travis.yml

@@ -23,7 +23,6 @@ env:
     global:
         # Make sure beaver is in the PATH
         - PATH="$(git config -f .gitmodules submodule.beaver.path)/bin:$PATH)"
-        - BEAVER_DOCKER_VARS="MXNET_ENGINE_TYPE"
 
 # Basic config is inherited from the global scope
 jobs:
@@ -39,32 +38,16 @@ jobs:
     include:
         # Test matrix
         - <<: *test-matrix
-          env: DMD=dmd1 DIST=xenial F=devel MXNET_ENGINE_TYPE=NaiveEngine
-        - <<: *test-matrix
-          env: DMD=dmd1 DIST=xenial F=devel MXNET_ENGINE_TYPE=ThreadedEngine
-        - <<: *test-matrix
-          env: DMD=dmd1 DIST=xenial F=devel MXNET_ENGINE_TYPE=ThreadedEnginePerDevice
+          env: DMD=dmd1 DIST=xenial F=devel
 
         - <<: *test-matrix
-          env: DMD=dmd1 DIST=xenial F=production MXNET_ENGINE_TYPE=NaiveEngine
-        - <<: *test-matrix
-          env: DMD=dmd1 DIST=xenial F=production MXNET_ENGINE_TYPE=ThreadedEngine
-        - <<: *test-matrix
-          env: DMD=dmd1 DIST=xenial F=production MXNET_ENGINE_TYPE=ThreadedEnginePerDevice
+          env: DMD=dmd1 DIST=xenial F=production
 
         - <<: *test-matrix
-          env: DMD=dmd-transitional DIST=xenial F=production MXNET_ENGINE_TYPE=NaiveEngine
-        - <<: *test-matrix
-          env: DMD=dmd-transitional DIST=xenial F=production MXNET_ENGINE_TYPE=ThreadedEngine
-        - <<: *test-matrix
-          env: DMD=dmd-transitional DIST=xenial F=production MXNET_ENGINE_TYPE=ThreadedEnginePerDevice
+          env: DMD=dmd-transitional DIST=xenial F=devel
 
         - <<: *test-matrix
-          env: DMD=dmd-transitional DIST=xenial F=devel MXNET_ENGINE_TYPE=NaiveEngine
-        - <<: *test-matrix
-          env: DMD=dmd-transitional DIST=xenial F=devel MXNET_ENGINE_TYPE=ThreadedEngine
-        - <<: *test-matrix
-          env: DMD=dmd-transitional DIST=xenial F=devel MXNET_ENGINE_TYPE=ThreadedEnginePerDevice
+          env: DMD=dmd-transitional DIST=xenial F=production
 
         # Additional stages