Drop builtin command documentation

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

Source/cmAddDefinitionsCommand.h

@@ -43,35 +43,6 @@ public:
    */
   virtual const char* GetName() const {return "add_definitions";}
 
-  /**
-   * Succinct documentation.
-   */
-  virtual const char* GetTerseDocumentation() const
-    {
-    return "Adds -D define flags to the compilation of source files.";
-    }
-
-  /**
-   * More documentation.
-   */
-  virtual const char* GetFullDocumentation() const
-    {
-    return
-      "  add_definitions(-DFOO -DBAR ...)\n"
-      "Adds flags to the compiler command line for sources in the current "
-      "directory and below.  This command can be used to add any flags, "
-      "but it was originally intended to add preprocessor definitions.  "
-      "Flags beginning in -D or /D that look like preprocessor definitions "
-      "are automatically added to the COMPILE_DEFINITIONS property for "
-      "the current directory.  Definitions with non-trivial values may be "
-      "left in the set of flags instead of being converted for reasons of "
-      "backwards compatibility.  See documentation of the directory, "
-      "target, and source file COMPILE_DEFINITIONS properties for details "
-      "on adding preprocessor definitions to specific scopes and "
-      "configurations."
-      ;
-    }
-
   cmTypeMacro(cmAddDefinitionsCommand, cmCommand);
 };