[cmake] Do not generate git_ignored_files.txt

Summary: It is completely unecessary to write this file on disk and read it back again.

Test Plan:

ninja

Added a message to check that _git_ignored_files indded contained the expected list.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5021