took out f-strings to be python2.7 compatible

bilby/core/sampler/kombine.py

@@ -132,7 +132,7 @@ class Kombine(Emcee):
             self.sampler.burnin(**self.sampler_burnin_kwargs)
             self._set_pos0_for_resume()
             self.nburn = self._previous_iterations
-            logger.info(f"Kombine auto-burnin complete. Removing {self.nburn} samples from chains")
+            logger.info("Kombine auto-burnin complete. Removing {} samples from chains".format(self.nburn))
 
         tqdm = get_progress_bar()
         sampler_function_kwargs = self.sampler_function_kwargs