imap-send: remove dead code

imap-send: remove dead code

The imap-send code was adapted from another project, and
still contains many unused bits of code. One of these bits
contains a type "struct string_list" which bears no
resemblence to the "struct string_list" we use elsewhere in
git. This causes the compiler to complain if git's
string_list ever becomes part of cache.h.

Let's just drop the dead code.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>