Drop builtin command documentation

Drop all GetTerseDocumentation and GetFullDocumentation methods from
commands.  The command documentation is now in Help/command/*.rst files.

Source/cmMathCommand.h

@@ -43,28 +43,6 @@ public:
    */
   virtual const char* GetName() const { return "math";}
 
-  /**
-   * Succinct documentation.
-   */
-  virtual const char* GetTerseDocumentation() const
-    {
-    return "Mathematical expressions.";
-    }
-
-  /**
-   * More documentation.
-   */
-  virtual const char* GetFullDocumentation() const
-    {
-    return
-      "  math(EXPR <output variable> <math expression>)\n"
-      "EXPR evaluates mathematical expression and returns result in the "
-      "output variable. Example mathematical expression is "
-      "'5 * ( 10 + 13 )'.  Supported operators are "
-      "+ - * / % | & ^ ~ << >> * / %.  They have the same meaning "
-      " as they do in C code.";
-    }
-
   cmTypeMacro(cmMathCommand, cmCommand);
 protected: