refactor(variables.less): remove deprecated @line-height-computed variable

The @line-height-computed has no use other than specifying the line-height in pixels. So, to make LESS job easier (and ours, probably), this was deprecated and removed immediately.
style(mixins.less): use dashed variables in #gradient mixins

The gradient mixins were using camelcase-style for their variables @startColor and @endColor, which are now using dash-style naming: @start-color and @end-color.