Don't try to remove non-existing packages.

This especially matters when building using tagged APT snapshots.
I've just seen a 3.0~beta2 build fail with:

  Package 'linux-headers-686-pae' is not installed, so not removed
  Package 'linux-headers-amd64' is not installed, so not removed
  Package 'linux-headers-generic' is not installed, so not removed
  Package 'linux-headers' is not installed, so not removed
  E: Unable to locate package ^linux-source-*
  E: Couldn't find any package by glob '^linux-source-*'
  E: Couldn't find any package by regex '^linux-source-*'
  E: Unable to locate package dpatch
  E: Unable to locate package gcc-4.8
  E: Couldn't find any package by glob 'gcc-4.8'
  E: Couldn't find any package by regex 'gcc-4.8'
  E: Unable to locate package gcc-4.9
  E: Couldn't find any package by glob 'gcc-4.9'
  E: Couldn't find any package by regex 'gcc-4.9'
  E: Unable to locate package gcc-5
  E: Unable to locate package gcc-5-base
  E: Unable to locate package libgl1-mesa-dev