Theme - Cleanup (btn-default)

```border-color``` is not nessessary. The color will be defined in the https://github.com/twbs/bootstrap/blob/master/less/buttons.less#L60.
```
.btn-default {
  .button-variant(@btn-default-color; @btn-default-bg;
@btn-default-border);
}
```

PS: I would also set the ```text-shadow``` to ```none``` or use the invert color of the ```@btn-default-color```. If you change the theme to a dark skin, the buttons text shadow will not look very nice. What do you think about it?