Removed is_sparse_array from objToJSON
@jbrockmendel @jorisvandenbossche 

i couldve sworn i did that in the other PR.  did i screw up? Are you thinking #29989 or another PR? If former I don't see this module touched in there; know we discussed it I assumed left to follow up #29989, yah, i even commented here https://github.com/pandas-dev/pandas/pull/29989#issuecomment-561874530 that i had removed is_sparse_array mystery notwithstanding, LGTM thanks @WillAyd 
do we actually tests that .to_json() with EAs? is this supported in any way currently?