Use unitless line-height

[ntwb]

Via WordPress/gutenberg#571

See https://make.wordpress.org/core/handbook/best-practices/coding-standards/css/#values

"Line height should also be unit-less, unless necessary to be defined as a specific pixel value. This is more than just a style convention, but is worth mentioning here. More information: http://meyerweb.com/eric/thoughts/2006/02/08/unitless-line-heights/"

Also https://developer.mozilla.org/en-US/docs/Web/CSS/line-height#Prefer_unitless_numbers_for_line-height_values

[ntwb]

Notes:

bbPress, BuddyPress, and BuddyPress' Nouveau all contain instances of line-height containing units, before merging this, this should be researched and tested before merging.