Plugin dovecot: remove usage of "expr"

The usage of "expr" can be replaced easily with a shell builtin.
"expr" caused problems due to its hard-coded path.

Closes: #1240

plugins/dovecot/dovecot

@@ -30,7 +30,6 @@
 ######################
 # Configuration
 ######################
-EXPR_BIN=/usr/bin/expr
 LOGFILE=${logfile:-/var/log/mail.log}
 ######################
 
@@ -75,7 +74,7 @@ echo -n
 ######################
 DISCONNECTS=$(egrep -c '[dovecot]?.*Disconnected' $LOGFILE)
 CONNECTS=$(egrep -c '[dovecot]?.*Login' $LOGFILE)
-VALUE=$($EXPR_BIN $CONNECTS - $DISCONNECTS)
+VALUE=$(( CONNECTS - DISCONNECTS ))
 if [ -z "$VALUE" ] || [ "$VALUE" -lt 0 ]; then
 	VALUE=0
 fi