Use absolute path for xterm on Mac and remove unused file.

Use absolute path for xterm on Mac and remove unused file.

xterm is (no longer?) in the PATH on Mac, so we need to point to it
directly.

Task-number: QTCREATORBUG-6150
Change-Id: I84202d0ebac1b8875c890116cce960898f65e5f9
Reviewed-on: http://codereview.qt-project.org/5303
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>