r3.forestfrag: Make string replace Python 3 compatible (#466)

This does for r3.forestfrag what 44ebd8f does for r.forestfrag.
Unused imports are removed also from r.forestfrag.

Removes unused flag -s (show r.report output) is from r3.forestfrag.

grass7/raster/r.forestfrag/r.forestfrag.py

@@ -100,7 +100,6 @@
 import uuid
 import atexit
 import tempfile
-import string
 import grass.script as gs
 # neutral naming for better compatibility between 2D and 3D version
 from grass.script.raster import mapcalc

grass7/raster3d/r3.forestfrag/r3.forestfrag.py

@@ -111,23 +111,15 @@
 #% description: Keep Pf and Pff maps
 #%end
 
-#%flag
-#% key: s
-#% description: Run r.report on output map
-#%end
-
 #%flag
 #% key: a
 #% description: Trim the output map to avoid border effects
 #%end
 
 
-import os
 import sys
 import uuid
 import atexit
-import tempfile
-import string
 import grass.script as gs
 # neutral naming for better compatibility with 2D version
 from grass.script.raster3d import mapcalc3d as mapcalc
@@ -200,7 +192,7 @@ def tmpname(prefix):
     Use only for raster maps.
     """
     tmpf = prefix + str(uuid.uuid4())
-    tmpf = string.replace(tmpf, '-', '_')
+    tmpf = tmpf.replace('-', '_')
     CLEAN_RAST.append(tmpf)
     return tmpf
 
@@ -264,7 +256,6 @@ def main(options, flags):
     user_pf = options['pf']
     user_pff = options['pff']
     flag_r = flags['r']
-    flag_s = flags['s']
     clip_output = flags['a']
 
     # set to current input map region if requested by the user