Make fonts always relative to project.less is not good idea

Created by: normanzb

veto to #10941 , the guy report the issue must because he is compiling with --relative-urls turned on, that's why fonts folder are relative to bootstrap.less

whether or not relative should be controlled by lessc parameter --relative-urls not by hard coding it.

it would be actually good idea to make sure fonts are relative to bootstrap.less (when --relative-urls is used) as i don't have to copy over the fonts from bower folder to project folder, a simple bower install should do the job.

@jhnns also mentioned that the change cause rootpath options useless in the thread of #10941 .