Fix Yosys DFF mapping (#875)

Remove `-map-only` flag that was breaking synthesis, causing many DFFs
to be emitted as Yosys internal abstract cells instead of being mapped
to the available DFF cells.

hammer/synthesis/yosys/__init__.py

@@ -270,7 +270,7 @@ def syn_generic(self) -> bool:
         # Technology mapping of flip-flops
         """)
         for liberty_file in self.liberty_files_tt.split():
-            self.verbose_append(f"dfflibmap -map-only -liberty {liberty_file}")
+            self.verbose_append(f"dfflibmap -liberty {liberty_file}")
         self.verbose_append("opt")
 
         self.write_sdc_file()