Fix tiny CSS glitches on segment editor page

This was causing small items like badges and values in multi selelect
to look broken.
[MAILPOET-3911]

mailpoet/assets/css/src/mailpoet-dynamic-segments.scss

@@ -1,10 +1,6 @@
 @import 'settings/colors';
 @import 'mailpoet-listing';
 
-#dynamic_segments_container * {
-  box-sizing: border-box;
-}
-
 .mailpoet-segment-subscriber-count {
   align-items: center;
   display: flex;