Miscellaneous "detheming" of the bp-legacy stylesheet

When introduced in BP 1.7, the bp-legacy stylesheet was mostly copied over from
bp-default, with some of the more global declarations removed. However, much of
bp-default's personality could still be found in bp-legacy, in the form of
button styling, font families, rounded corners, and other such details.

This changeset removes some of these remnants of bp-default's styling, so that
sites using theme compatibility will inherit details more elegantly from the
active theme. The result is a much more consistent visual experience, with
buttons and other BP elements matching the rest of the theme more
seamlessly.

Fixes #4953

Props karmatosed, hnla

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7206 cdf35c40-ae34-48e0-9cc9-0c9da1808c22