Revert "Enable D8 dex merging (oppia#3014)" (oppia#3023)

This reverts commit f81d8a3.

.bazelrc

@@ -3,7 +3,7 @@ build --android_databinding_use_v3_4_args \
  --experimental_android_databinding_v2 \
  --java_header_compilation=false \
  --noincremental_dexing \
- --nouse_workers_with_dexbuilder
+ --define=android_standalone_dexing_tool=d8_compat_dx
 
 # Show all test output by default (for better debugging).
 test --test_output=all