Mattermost-server: Webapp: Remove border on code blocks

[last-partizan]

When using dark theme light gray border on code blocks looks a bit off.

It can be fixed either by removing border in .post code code, or by adding dark border in dark colorschemes for code.

[amyblais]

@last-partizan Would you be interested in opening a pull request for this for devs to review?

[last-partizan]

@amyblais yes, i can do it.

[lindalumitchell]

@amyblais do we need a JIRA ticket to track this as well? I didn't see one, but wasn't sure if this is on a different process.

[amyblais]

Ticket: https://mattermost.atlassian.net/browse/MM-18615.