Remove unneeded QWP_PATH environment variable for quick tests.

When the PATH is properly set up, this should not be necessary
and it overrode the explicitly set QTWEBENGINEPROCESS_PATH.

Change-Id: Ie83a9f4f15f2203329c44fd9653adbb19c62672c
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>