Remove the AttrBuilder version of the Attribute::get function.

The AttrBuilder is there to build up multiple attributes. The Attribute class
represents only one attribute at a time. So remove this unnecessary builder
creator method.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174010 91177308-0d34-0410-b5e6-96231b3b80d8