Search your Field Editor files for this line:
I believe it's in mcp.field_editor.php and libraries/Package_installer.php. Delete those lines. It's not necessary for newer versions of EE.
I do not know if there are any other issues with Field Editor and EE 2.8, but this should clear up this particular ...
Sounds like you need to reset the Publish Layout. Select the member groups for which the layout should be reset and click the "Reset Layout" button. Then refresh the browser window. You'll then need to rebuild your Publish Layout.
I think it will be enough to delete field record in exp_channel_fields table.
Go to EE CP SQL manager (CP Home > Tools > Data > SQL Manager > Database Query Form) or phpMyAdmin or any other mysql manager:
DELETE FROM exp_channel_fields where field_id = 21;
if you want to be sure that any pieces of your grid field was removed, execute query bellow line-by-...
There's a dropdown menu on Playa's specific Add/Edit Channel Field ( Admin > Channel Fields > Group > Field ) form called Limit entries to - this is where you can set a limit.
Any searches/filters on the playa field will search/filter the all entires ( configured in the CP ) and not just filtered set.