[TYPING](ALL): Fix typing errors

biobb_pmx/pmxbiobb/pmxmerge_ff.py

@@ -8,7 +8,6 @@
 import argparse
 from typing import Optional
 from pmx import ligand_alchemy  # type: ignore
-from typing import Optional
 from biobb_common.generic.biobb_object import BiobbObject
 from biobb_common.configuration import settings
 from biobb_common.tools import file_utils as fu