lower_case_table_names = 0 # default value for unix
Change lower_case_table_names in my.cnf to
lower_case_table_names = 1
The error message comes from unix rmdir himself.
All *.ibd and db.opt already deleted, but files T1.frm still exists in database directory,
which provoke above error message.
DROP DATABASE should check, if some .frm names are in the datadir with capital_letters,
if lower_case_table_names = 1
2 Solutions are possible.
1. Throw an error before ! deleting *.ibd files
2. Delete also the .frm files with Captial letter.