convert: remove unused and incorrect default handling for revmapfile

destc is not a string and can thus not be os.path.join'ed. Convert would crash
if we ended up there ... but we wouldn't because both the sinks (hg and
subversion) sinks implement .revmapfile and "never" throws exceptions.

hgext/convert/convcmd.py

@@ -516,10 +516,7 b' def convert(ui, src, dest=None, revmapfi'
         destc.setfilemapmode(True)
 
     if not revmapfile:
-        try:
-            revmapfile = destc.revmapfile()
-        except Exception:
-            revmapfile = os.path.join(destc, "map")
+        revmapfile = destc.revmapfile()
 
     c = converter(ui, srcc, destc, revmapfile, opts)
     c.convert(sortmode)