Fix debug environment setting

Debug environment value was replaced by default value on every build
or debugging session. It was caused by a try to remove illegal value
from project user file. Implementation was not working as expected and
is totally removed now. It is unnecessary because problem is caused by
addin version 1.1.11 supporting Qt4 and only Qt5 support is available
in 1.2.x versions. Also just removing project user file solves the
issue of illegal value if it exist.

Task-number: QTVSADDINBUG-159
Change-Id: I420a97c3061e1908856ce8da187e28cf0e45c63f
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>