lib/history: remove redundant export for HISTIGNORE (#501)

There does not seem to be no specific reason that we export HISTIGNORE
[1].  These lines were introduced in commit 53fb803 but only
HISTIGNORE had `export'.

[1] #500 (comment)

lib/history.sh

@@ -36,7 +36,7 @@ export HISTFILESIZE=
 HISTCONTROL="erasedups:ignoreboth"
 
 # Don't record some commands
-export HISTIGNORE="exit:ls:bg:fg:history:clear"
+HISTIGNORE="exit:ls:bg:fg:history:clear"
 
 # Enable incremental history search with up/down arrows (also Readline goodness)
 # Learn more about this here: https://codeinthehole.com/tips/the-most-important-command-line-tip-incremental-history-searching-with-inputrc/