removing empty directories search after each ports

[olevole]

Removed without much sense line that search for empty directories in WRKDIRPREFIX after each port. If you define WRKDIRPREFIX (such as /tmp) and portmaster execute with -C and -K flags, this procedure greatly increases the time portmaster because /tmp/usr/ports have are large structures while the deletion of empty directories there is no benefit

[bdrewery]

Related to #6. It would be better to make this optional instead of removing it completely.

[p]

:+1: for the option.