[Releasing] Remove "branch" lines from .gitmodules in qt5.git prior to tagging

[Thiago Macieira]

The .gitmodule file contains "branch" lines that are invalid:

[submodule "qtbase"]
        path = qtbase
        url = ../qtbase.git
        branch = 5.7.1
        status = essential

Please remove those lines as the branch gets deleted too after the release.

This started happening in the 5.4.0 release. 5.3.2 is not affected.
-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center