Remove 'git checkout 5.*' before 'init-repository'.

Something has changed in Qt libraries remote layout so now the perl
init-repository script from the old Qt version doesn't work anymore.

We init the submodules using script from the latest Qt version and
after that we checkout the desired version by tag in all submodules.

docs/building-cmake.md

@@ -133,7 +133,6 @@ In Terminal go to **/home/user/TBuild/Libraries** and run
 
     git clone git://code.qt.io/qt/qt5.git qt5_6_2
     cd qt5_6_2
-    git checkout 5.6
     perl init-repository --module-subset=qtbase,qtimageformats
     git checkout v5.6.2
     cd qtimageformats && git checkout v5.6.2 && cd ..

docs/building-msvc.md

@@ -191,7 +191,6 @@ While still running the VS2015 x86 Native Tools Command Prompt, go to **D:\\msys
 
     git clone git://code.qt.io/qt/qt5.git qt5_6_2
     cd qt5_6_2
-    git checkout 5.6
     perl init-repository --module-subset=qtbase,qtimageformats
     git checkout v5.6.2
     cd qtimageformats && git checkout v5.6.2 && cd ..

docs/building-qmake.md

@@ -38,7 +38,6 @@ Preparation
       rm -rf "$qt_dir"
       git clone git://code.qt.io/qt/qt5.git
       cd "$qt_dir"
-      git checkout 5.6
       perl init-repository --module-subset=qtbase,qtimageformats
       git checkout v$_qtver
       cd qtimageformats

docs/building-qtcreator.md

@@ -129,7 +129,6 @@ In Terminal go to **/home/user/TBuild/Libraries** and run
 
     git clone git://code.qt.io/qt/qt5.git qt5_6_2
     cd qt5_6_2
-    git checkout 5.6
     perl init-repository --module-subset=qtbase,qtimageformats
     git checkout v5.6.2
     cd qtimageformats && git checkout v5.6.2 && cd ..

docs/building-xcode-old.md

@@ -184,7 +184,6 @@ In Terminal go to **/Users/user/TBuild/Libraries** and run:
 
     git clone git://code.qt.io/qt/qt5.git qt5_3_2
     cd qt5_3_2
-    git checkout 5.3
     perl init-repository --module-subset=qtbase,qtimageformats
     git checkout v5.3.2
     cd qtimageformats && git checkout v5.3.2 && cd ..

docs/building-xcode.md

@@ -182,7 +182,6 @@ In Terminal go to **/Users/user/TBuild/Libraries** and run:
 
     git clone git://code.qt.io/qt/qt5.git qt5_6_2
     cd qt5_6_2
-    git checkout 5.6
     perl init-repository --module-subset=qtbase,qtimageformats
     git checkout v5.6.2
     cd qtimageformats && git checkout v5.6.2 && cd ..