Use unit-less values for line-height

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/

