Tag suggest suggests Terms from other taxonomies than post_tag

[DD32]

To reproduce:

• Create a category 'Random Cat'
• Create a tag 'Random Tag'
• Start to add a Tag to the post, Type 'Ran', Observe both the Cat and Tag showing up.

The attached patch limits the tag suggest to the 'post_tag' taxonomy

Note: Even if you select a Cat, and attempt to add it to the tag list, While the UI shows its added on the post screen, Upon a refresh, it has not been added to either the Tag list or the Tag list.

[DD32]

(Note, This has been an issue since the Tag suggest feature was introduced in 2.5, [6542], Setting version to reflect that)

[ryan]

(In [9629]) Limit tag suggest to tags. Props DD32. fixes #8164