checkout: remove duplicate code

checkout: remove duplicate code

Both commit a7256debd4b6 ("checkout.txt: note about losing staged
changes with --merge", 2019-03-19) from nd/checkout-m-doc-update and
commit 6eff409e8a76 ("checkout: prevent losing staged changes with
--merge", 2019-03-22) from nd/checkout-m were included in git.git
despite the fact that the latter was meant to be v2 of the former.
The merge of these two topics resulted in a redundant chunk of code;
remove it.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>