Issue 6577: remove confirmation to delete output directory

Issue 6577: remove confirmation to delete output directory

Change-Id: Ib19986121a8988c6cae23527148a7b5d4a58663e

BUG=chromium-os:6577
TEST=(inside chroot) run 4 test cases from src/scripts/, using Ctrl+C to force abort (hence triggering "delete_prompt")
a.) (stdin tty) "./build_image --board=x86-generic < /dev/null", expected = no prompt and delete output dir; actual == expected
b.) (stdout tty) "./build_image --board=x86-generic > foo.txt", expected = no prompt and delete output dir; actual == expected
c.) (normal user case) "./build_image --board=x86-generic", 
expected = prompt to delete output dir, if y, output dir is removed; actual == expected
d.) (normal user case) "./build_image --board=x86-generic", expected = prompt to delete output dir, if N, output dir is NOT removed; actual == expected

[sosa, 2010-09-09 17:57:22]

I don't like changing something that is useful for manual people for the
purposes of testing.  We should add a flag for testing to bypass the check, not
the other way around.  I've really found this useful in my own work.  I never
want the output dir around and doing $(./get_latest_image) doesn't work so I"d
have to manually rm -rf the dir after finding it which takes forever

[Tan Gao, 2010-09-09 18:35:24]

understood :-) CL updated. PTAL thx!

[sosa, 2010-09-09 18:40:19]

LGTM, thanks to both of you for compromising with me :D