fix: Enable libsass#2920

[skip libsass]

Author: nschonni

spec/core_functions/string/slice.hrx

@@ -158,11 +158,6 @@ a {
 
 <===>
 ================================================================================
-<===> start/negative/after_last/options.yml
----
-:todo:
-- sass/libsass#2920
-
 <===> start/negative/after_last/input.scss
 a {b: str-slice("cde", -100)}