Remove "port -u uninstall" functionality

[ryandesign (Ryan Carsten Schmidt)]

The global -u flag has two unrelated uses:

1. "sudo port -u upgrade foo" means upgrade port foo to the latest version and uninstall all other versions of foo.
2. "sudo port -u uninstall" means uninstall all inactive ports; it's equivalent to "sudo port uninstall inactive".

I propose that the second meaning of -u be removed from MacPorts.