removed AVX2 from VS config and reset PlatformToolset values back

[StrikerRUS]

For the reason of removing AVX2 refer to #2699 (comment).

Reset PlatformToolset values back to allow compilation with old versions of MSBuild.
Current behavior after #2699 at Appveyor (image: Visual Studio 2015):

INFO:LightGBM:Starting to compile the library.
INFO:LightGBM:Starting to compile with MSBuild from existing solution file.
WARNING:LightGBM:Compilation with MSBuild from existing solution file failed.
INFO:LightGBM:Starting to compile with Visual Studio 16 2019.
INFO:LightGBM:Starting to compile with Visual Studio 15 2017.
INFO:LightGBM:Starting to compile with Visual Studio 14 2015.
running build
running build_py
creating build

Newer versions should automatically update that values, or we have guide how to change them manually: https://lightgbm.readthedocs.io/en/latest/Installation-Guide.html#with-gui.

[guolinke]

sorry, I forgot to update it in my PR