Remove SH_NOLOG vestiges

The '-o nolog' option (which prevented function definitions from being
recorded in the history file) was removed a long time ago, leaving
only a stub for backwards compatibility to stop 'set' from erroring
out if the option is set. But some other vestiges remained.

src/cmd/ksh93/sh/path.c,
src/cmd/ksh93/sh/subshell.c,
src/cmd/ksh93/sh/xec.c:
- Remove a few pointless 'sh_onstate(SH_NOLOG)' statements. As of
  93u+ or earlier, this is never checked for anywhere.

src/cmd/ksh93/sh.1:
- They forgot to remove the 'nolog' option documentation here.
  Specify that it's obsolete and has no effect.

src/cmd/ksh93/data/builtins.c: sh_set[]:
- Be more concise.

src/cmd/ksh93/sh/subshell.c

@@ -206,7 +206,6 @@ void sh_subfork(void)
 		/* this is the child part of the fork */
 		/* setting subpid to 1 causes subshell to exit when reached */
 		sh_onstate(SH_FORKED);
-		sh_onstate(SH_NOLOG);
 		sh_offoption(SH_MONITOR);
 		sh_offstate(SH_MONITOR);
 		subshell_data = 0;