Remove Go Attribute type that was deleted from the C API in r286062.

Summary:
The C++ side of the Go bindings were updated in r286085, r286086, and r286087,
but those did not remove this type.

Reviewers: djasper

Subscribers: axw

Differential Revision: https://reviews.llvm.org/D26337

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286131 91177308-0d34-0410-b5e6-96231b3b80d8

bindings/go/llvm/ir.go

@@ -58,7 +58,6 @@ type (
 	Metadata struct {
 		C C.LLVMMetadataRef
 	}
-	Attribute        uint64
 	Opcode           C.LLVMOpcode
 	TypeKind         C.LLVMTypeKind
 	Linkage          C.LLVMLinkage