Remove LGPL_EXCEPTION.txt

It became obsolete when we switched to LGPL3, as discussed on the
developer mailing list in April 2017.

Change-Id: I7364ce547f952380d8b8e32b89c09b6ace3c9188
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

LGPL_EXCEPTION.txt

-The Qt Company Qt LGPL Exception version 1.1
-
-As an additional permission to the GNU Lesser General Public License version
-2.1, the object code form of a "work that uses the Library" may incorporate
-material from a header file that is part of the Library.  You may distribute
-such object code under terms of your choice, provided that:
-    (i)   the header files of the Library have not been modified; and 
-    (ii)  the incorporated material is limited to numerical parameters, data
-          structure layouts, accessors, macros, inline functions and
-          templates; and
-    (iii) you comply with the terms of Section 6 of the GNU Lesser General
-          Public License version 2.1.
-
-Moreover, you may apply this exception to a modified version of the Library,
-provided that such modification does not involve copying material from the
-Library into the modified Library's header files unless such material is
-limited to (i) numerical parameters; (ii) data structure layouts;
-(iii) accessors; and (iv) small macros, templates and inline functions of
-five lines or less in length.
-
-Furthermore, you are not required to apply this additional permission to a
-modified version of the Library.