Retire support for old environment variables.

We have deprecated the old environment variable names for quite a
while and now it's time to remove them.  Gone are:

SHA1_FILE_DIRECTORIES AUTHOR_DATE AUTHOR_EMAIL AUTHOR_NAME
COMMIT_AUTHOR_EMAIL COMMIT_AUTHOR_NAME SHA1_FILE_DIRECTORY

Signed-off-by: Junio C Hamano <junkio@cox.net>