We may be able to remove it entirely. The only bbcode that makes use of the <p> tag is [code]. The only *CP templates that make use of .postbody are the following. Uses of the <p> tag within .postbody are noted next to the file.
mcp_post.html [p.author, p.post-notice.reported, p.post-notice.unapproved, p.post-notice.deleted]
mcp_topic.html [p.author, p.post-notice.reported, p.post-notice.unapproved, p.post-notice.deleted]
From the <p> uses above, only p.author seems to be affected by the removal of the style. So, we should be able to move the font-size property under a new selector targetting p.author for *CP pages (#cp-main .postbody .author).