PATCH: To fix the performance of delete operation (pgAdmin4)

PFA patch to fix the issue of slow performance while deleting rows from grid.

RM#1696

As per logic implemented for Backgrid, we were deleting individual rows, means If user selects 100 rows to delete then we were rendering sql template 100 times to generate sql for each individual rows and then execute all 100 delete statements one by one due to which we were getting poor performance.

Please review.