Back compat issues in delete(d)_postmeta actions

[duck_]

I was looking at #18823 and noticed some backwards compatibility issues with the delete(d)_postmeta actions in 3.3.

See [18500] and #18196 for what looks like accidental backwards incompatibility.

• The deleted_postmeta action disappeared (see delete_metadata_by_mid); this was probably a copy and paste error.
• delete_meta( $mid ) will now run the delete_postmeta action with the post ID ($object_id) instead of the $mid.

Patch attached fixes these up to be backwards compatible with s/$object_id/$meta_id/ and adding the missing 'd'.

Looks good.

In [18835]:

Fix back compat issues with delete_postmeta and deleted_postmeta actions as these should be passed the meta ID. Fixes #18825.