If you aren't using Poll feature you can delete the Poll folder and delete PollFeature.Business.dll, PollFeature.Data.dll, and PollFeature.UI.dll from the bin folder and you can delete the setup folder /Setup/applications/pollfeature-byChristianFredh
Same with Survey and WebStore, you can delete the folders, corresponding dlls and setup files.
Additionally, you can delete the sql scripts for dbs you are not using from under /Setup/applications/mojoportal-core/SchemaInstallScripts and from /SchemaUpgradeScripts. ie if you are using MSSQL, you can delete the ones for MySql, Pgsql, Firebird and sqlite.
You can also delete the SiteOffice folder.
If you've been using mojoportal for a long time you may also have some old versions of some javascript you can delete from /ClientScript, like older versions of extjs or fckeditor, and also if you see a folder for dojo you can delete that as its not been used or included for a long time.
Hope it helps,
Joe