variables.less: Removed the comments following @font-size-hN definitions...

Rationale: I've upgraded Bootstrap versions many times before, for several projects.

The comments that accompany the `@font-size-hn` variables are dependent on the  `@font-size-base` value.

I suggest removing these because:
 
 * the `@font-size-base` value is among the likeliest to change upon customization,
 * the font-size progression just isn't difficult to grok,
 * I can't imagine these comments in `variables.less` are useful to anyone.

At least, I've never found these `@font-size-hn` comments to be useful other than being incorrect quite soon in the customization process.