removed all caret border color settings.

In most cases, `caret` color should be same as text color.

You tried to get this by setting border-color in many places to the color of text. But not in all cases. For instance for buttons with other state than the default state, it is set to `#fff` instead of using color-variables of states and handling for `:hover` I missing completely.

But for what all this overhead?

Why you do not use the default behavior of border-color? The default is, if it is not specified, the text color will be used. And that is exact what we want, isn't?

In this pull request I removed all the border-color setting for `caret`.

Please give it a try.

It would simplify the Bootstrap CSS and give everybody the chance, to set a caret color by simply set the `color` property for class `.caret`.