Revert "Add GetModifiedFiles() function to git_utils."

This reverts commit af1de867422c42855ae54ed3838c62ea445ea6b4.

After removing svndiff.py in
https://skia.googlesource.com/skia/+/70242ace3b2ed06b33ee5ea5c9ac3aedea89b194,
nobody uses this function anymore.

R=borenet@google.com

Review URL: https://codereview.chromium.org/655383002