Revert "Let travis compile debug binaries so ASSERTS are also considered"

This reverts commit b1de41c.

Led to unstable test results for no reason

See

https://travis-ci.org/qgis/QGIS/builds/42811273

Different results for the very same commit:
http://dash.orfeo-toolbox.org/buildSummary.php?buildid=165433
http://dash.orfeo-toolbox.org/buildSummary.php?buildid=165427

Author: m-kuhn

.travis.yml

@@ -18,7 +18,7 @@ before_install:
 install:
   - mkdir build
   - cd build
-  - cmake -DCMAKE_BUILD_TYPE=Debug -DWITH_SERVER=ON -DWITH_STAGED_PLUGINS=OFF -DWITH_GRASS=OFF -DSUPPRESS_QT_WARNINGS=ON ..
+  - cmake -DWITH_SERVER=ON -DWITH_STAGED_PLUGINS=OFF -DWITH_GRASS=OFF -DSUPPRESS_QT_WARNINGS=ON ..
 
 script: xvfb-run ctest -V -E 'PyQgsPalLabelingCanvas|PyQgsPalLabelingServer|qgis_wcsprovidertest' -S ../qgis-test-travis.ctest --output-on-failure