update docs and header for removed error message

doc/src/Errors_messages.txt

@@ -279,12 +279,6 @@ multibody joint).  The bodies you have defined exceed this limit. :dd
 This is an internal LAMMPS error.  Please report it to the
 developers. :dd
 
-{Atom sorting has bin size = 0.0} :dt
-
-The neighbor cutoff is being used as the bin size, but it is zero.
-Thus you must explicitly list a bin size in the atom_modify sort
-command or turn off sorting. :dd
-
 {Atom style hybrid cannot have hybrid as an argument} :dt
 
 Self-explanatory. :dd

doc/src/atom_modify.txt

@@ -166,7 +166,8 @@ info), a map is used.  The default map style is array if no atom ID is
 larger than 1 million, otherwise the default is hash.  By default, a
 "first" group is not defined.  By default, sorting is enabled with a
 frequency of 1000 and a binsize of 0.0, which means the neighbor
-cutoff will be used to set the bin size.
+cutoff will be used to set the bin size. If no neighbor cutoff is
+defined, sorting will be turned off.
 
 :line
 

src/atom.h

@@ -496,12 +496,6 @@ E: Atom sort did not operate correctly
 This is an internal LAMMPS error.  Please report it to the
 developers.
 
-E: Atom sorting has bin size = 0.0
-
-The neighbor cutoff is being used as the bin size, but it is zero.
-Thus you must explicitly list a bin size in the atom_modify sort
-command or turn off sorting.
-
 E: Too many atom sorting bins
 
 This is likely due to an immense simulation box that has blown up