Unify package removal code

Unify package removal code

This code duplication has always been a rather clumsy casuality of
fixing some past upgrade issues. Unify the removal code across upgrade
and remove operations into  a new _alpm_remove_single_package() method
wihch makes it very clear how we handle upgrade and remove differently,
via several conditionals on newpkg.

This commit highlights interesting behavior such as the fact that the
implicit removal in every package upgrade never gets transaction events
or progress callbacks.

Signed-off-by: Dan McGee <dan@archlinux.org>