Remove unused, or possibly deleterious, resets on `sup`, `sub` elements

These rules were added 10 years ago. We don't have any `sup` or `sub`
elements in our source, and they could have possibly deleterious effects
when user content is styled.

Part of https://github.com/hypothesis/frontend-shared/issues/208

src/styles/reset.scss

@@ -20,20 +20,6 @@ body {
   margin: 0;
 }
 
-sub,
-sup {
-  font-size: 75%;
-  line-height: 0;
-  position: relative;
-}
-
-sup {
-  top: -0.5em;
-}
-sub {
-  bottom: -0.25em;
-}
-
 pre {
   white-space: pre;
   white-space: pre-wrap;