Remove old files from the requirements list

global-requirements.txt is the only file that needs to be processed by
the first part of this job now. Stop trying to read the files that we're not
going to sync from. dev-requirements.txt also gets processed, but later.

Change-Id: I29e8c381725b7e0b70bad7312ab1f05189cb3fc4
Merge "Remove old files from the requirements list"