entry.c: remove "checkout-index" from error messages

entry.c: remove "checkout-index" from error messages

Back then when entry.c was part of checkout-index (or checkout-cache
at that time [1]). It makes sense to print the command name in error
messages. Nowadays entry.c is in libgit and can be used by any
commands, printing "git checkout-index: blah" does no more than
confusion. The error messages without it still give enough information.

[1] 12dccc1 (Make fiel checkout function available to the git library - 2005-06-05)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>