u-string-list: move "remove duplicates" test to "u-string-list.c"

u-string-list: move "remove duplicates" test to "u-string-list.c"

We use "test-tool string-list remove_duplicates" to test the
"string_list_remove_duplicates" function. As we have introduced the unit
test, we'd better remove the logic from shell script to C program to
improve test speed and readability.

As all the tests in shell script are removed, let's just delete the
"t0063-string-list.sh" and update the "meson.build" file to align with
this change.

Also we could simply remove "DISABLE_SIGN_COMPARE_WARNINGS" due to we
have already deleted related code.

Unfortunately, we cannot totally remove "test-string-list.c" due to that
we would test the performance of sorting about string list by executing
"test-tool string-list sort" in "p0071-sort.sh".

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