test-lib: retire $remove_trash variable

The convention "$remove_trash is set to the trash directory that is used during the test, so that it will be removed at the end, but under --debug option we set the varilable to empty string to preserve the directory" made sense back when it was introduced, as there was no $TRASH_DIRECTORY variable. These days, since no tests looks at the variable, it is obscure and even risks that by mistake the variable gets used for something else (e.g. remove_trash=yes) and cause us misbehave. Worse yet, remove_trash was not initialized to an empty string at the beginning, so a stray environment variable the user has could have affected the logic when "--debug" is in use. Rewrite the clean-up sequence in test_done helper to explicitly check the $debug condition and remove the trash directory using the $TRASH_DIRECTORY variable. Note that "go to the directory one level above the trash and then remove it" is kept and this is deliverate; test_at_end_hook_ will keep running from the expected location, and also some platforms may not like a directory that is serving as the $cwd of a still-active process removed. Signed-off-by: Junio C Hamano <gitster@pobox.com>