chore: remove unused string-constant environment variables (#227)

MESSAGE, BRANCH, AUTHOR_NAME, AUTHOR_EMAIL, DRY_RUN, JOB_SUMMARY,
PR_COMMENTS, and PR_TITLE were defined via os.getenv() but never read
anywhere in the codebase. The actual logic uses env_flag() which reads
the environment variables again from scratch via string literals.

These 8 lines were pure dead code.

Author: shenxianpeng

main.py

@@ -13,15 +13,6 @@
 COMMIT_MESSAGE_DELIMITER = "\x00"
 COMMIT_SECTION_SEPARATOR = "\n---\n"
 
-# Environment variables
-MESSAGE = os.getenv("MESSAGE", "false")
-BRANCH = os.getenv("BRANCH", "false")
-AUTHOR_NAME = os.getenv("AUTHOR_NAME", "false")
-AUTHOR_EMAIL = os.getenv("AUTHOR_EMAIL", "false")
-DRY_RUN = os.getenv("DRY_RUN", "false")
-JOB_SUMMARY = os.getenv("JOB_SUMMARY", "false")
-PR_COMMENTS = os.getenv("PR_COMMENTS", "false")
-PR_TITLE = os.getenv("PR_TITLE", "false")
 GITHUB_STEP_SUMMARY = os.environ["GITHUB_STEP_SUMMARY"]