cue: remove support for legacy protobuf attributes

https://cue-review.googlesource.com/c/cue/+/9368 in April 2021
added a bit of code so that we could still support old attributes like

    @protobuf(4,type=double,name=double_value)

alongside the new form

    @protobuf(4,double,name=double_value)

It has been over three years now, and we are early in the v0.11
release cycle, so it is a good time to remove the compatibility code.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Iba2f1339a56138f642216bb802612fe8402f547b
Dispatch-Trailer: {"type":"trybot","CL":1200324,"patchset":1,"ref":"refs/changes/24/1200324/1","targetBranch":"master"}

cue/attribute.go

@@ -211,13 +211,5 @@ func (a *Attribute) Flag(pos int, key string) (bool, error) {
 // and reports the value if found. It reports an error if the attribute is
 // invalid or if the first pos-1 entries are not defined.
 func (a *Attribute) Lookup(pos int, key string) (val string, found bool, err error) {
-	val, found, err = a.attr.Lookup(pos, key)
-
-	// TODO: remove at some point. This is an ugly hack to simulate the old
-	// behavior of protobufs.
-	if !found && a.attr.Name == "protobuf" && key == "type" {
-		val, err = a.String(1)
-		found = err == nil
-	}
-	return val, found, err
+	return a.attr.Lookup(pos, key)
 }