Revert "Add IsAnnotated field to models.Tag struct"

It seems that `git for-each-ref` is a lot slower than `git tag --list` when
there are thousands of tags, so revert back to the previous method, now that we
no longer use the IsAnnotated field.

This reverts commit b12b1040c3.

pkg/commands/models/tag.go

@@ -3,13 +3,9 @@ package models
 // Tag : A git tag
 type Tag struct {
 	Name string
-
 	// this is either the first line of the message of an annotated tag, or the
 	// first line of a commit message for a lightweight tag
 	Message string
-
-	// true if this is an annotated tag, false if it's a lightweight tag
-	IsAnnotated bool
 }
 
 func (t *Tag) FullRefName() string {