Remove outdated flex comments + other small changes

[Starsam80]

First commit just removes some stuff I found that related to the flexbox change. When I searched for these files, I couldn't find any references to them, so I believe that is safe to remove them.

The second commit cleans up _spacing.scss a little bit:

• Adjust whitespace a little bit
• Use a single @include media-breakpoint-up for all the classes
• Rearrange the margin: auto classes so it's the same pattern (all, top, right, bottom, left, x, y)

The third commit changes the width and height classes to use variables, allowing people to add their own values if needed.