Remove fixed height from MR diff headers

What does this MR do?

• This MR removes the fixed 56px height on MR diff and commit file titles

Context

• The MR diff file header & commit file headers are 56px tall, while snippets and repo file headers are only 42px. Is there a reason for the fixed 56px height? It seems overly tall.

• I realize 42px isn't a part of our grid, but I propose we tackle that separately as changing the height will have effects on the vertical alignment/spacing of the text and buttons within those file headers.

Before	After

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/62596