Bundled Themes: Twenty Fifteen bulleted list block appears too far to the left in the editor.

Fixes the issue by removing unnecessary margin rules for bulleted lists in the editor.

Props kjellr.
Fixes #50029.
Built from https://develop.svn.wordpress.org/trunk@47934


git-svn-id: https://core.svn.wordpress.org/trunk@47707 1a063a9b-81f0-0310-95a4-ce76da25c4cd