Removes duplicate calls from `this.calling`

This code is useless because it is duplicated in `visitCall` (https://github.com/LearnBoost/stylus/blob/master/lib/visitor/evaluator.js#L348 and https://github.com/LearnBoost/stylus/blob/master/lib/visitor/evaluator.js#L384).

For example:
```
border-radius(x)
  -moz-border-radius: x
  border-radius: x

.a
  border-radius: 5px

.b
  border-radius: 10px
```

`this.calling` before patch:
```
[]
[ 'border-radius', 'border-radius' ]
[ 'border-radius', 'border-radius' ]
[]
[]
[ 'border-radius', 'border-radius' ]
[ 'border-radius', 'border-radius' ]
[]
```

`this.calling` after patch:
```
[]
[ 'border-radius' ]
[ 'border-radius' ]
[]
[]
[ 'border-radius' ]
[ 'border-radius' ]
[]
```