qsubprocess: don't pass BZR_PDB env variable to subprocess

[Alexander Belchenko]

While I've tried to debug problem of bug https://bugs.launchpad.net/qbzr/+bug/570564 I've encountered problem with qsubprocess: it seems BZR_PDB passed to subprocess and in some cases subprocess invokes pdb session while I need it for main process. After chat with Gary we agreed on the idea of clearing BZR_PDB variable from subprocess environment.

[10:36] <bialix> GaryvdM: any ideas re BZR_PDB in subprocess? maybe we should manually delete this variable from subprocess environment?
[10:36] <GaryvdM> That would be a good idea.
[10:37] <GaryvdM> And if one needs to debug the subprocess, then one can just run the subprocess.