Removed note regarding glm fork

3rdparty/glm-git-modified-readme.txt

-The folder glm-git-modified/ contains a slightly modified version of glm (https://github.com/g-truc/glm),
-cloned from git and based off the pre-version of 0.9.8.0.
-
-The modifications fix a problem that causes non-trivially-copyable types not be allowed as type T
-in glm's vector, matrix and quaternion types.
-A pull request has been proposed to glm: https://github.com/g-truc/glm/pull/543
-
-My modified version is available at https://github.com/patrikhuber/glm. We stay as close as possible
-to the original version and try to get these fixes into upstream glm.