Remove duplicate AttributeList::removeAttributes implementation

Have the AttributeList overload delegate to the AttrBuilder one.
Simplify the AttrBuilder overload by avoiding getSlotAttributes, which
creates temporary AttributeLists.

Simplify `AttrBuilder::removeAttributes(AttributeList, unsigned)` by
using getAttributes instead of manually iterating over slots.

Extracted from https://reviews.llvm.org/D32262

NFC

llvm-svn: 300863