Revert deprecation of amenity=dancing_school

[tyrasd]

See openstreetmap/iD#6251 (comment) and following comments.

This would involve the following changes:

• drop the deprecation of amenity=dancing_school
• change the "Dance School" preset to use amenity=dancing_school as its tag
• (optional) add a field to indicate that a "Dance Hall" also offers lessons (dance:teaching=yes)

[matkoniecz]

see also https://www.openstreetmap.org/changeset/114834704

[simonpoole]

General remark: could we be somewhat more conservative with "deprecating" stuff in general (in this case it was always clearly wrong). There is no agreed on process for deprecating tags, which does not mean that it is never a good idea, but simply something snuck in to a tagging proposal should not be enough. See also https://www.openstreetmap.org/user/Mateusz%20Konieczny/diary/398555#comment51838

[mnalis]

(ref: https://community.openstreetmap.org/t/list-of-deprecated-tags-linked-to-the-new/96737)