Add remove method to operate on AttrBuilder instead of AttributeSet.

Prior to this change we would have to construct a temporary AttributeSet (which isn't temporary at all given that its allocated on the context), just to contain the attributes in the builder, then call remove on that.

Now we can just remove any attributes from the (lightweight and really temporary) builder itself.

Will be used in a future commit to remove some temporary attributes sets.

llvm-svn: 236666