Clean up merge_target_files.py.

- Removes functions that can be replaced with one-line external methods
- Moves read_config_list to common alongside similar methods LoadDictionaryFrom*
- Runs pyformat on merge_target_files.py

Bug: 137853921
Test: python -m unittest test_merge_target_files
Test: Using merge_target_files.py to create a merged build, & booting.
Change-Id: I833c1086db41e1374057cc7447fc50d1915734a7