Just remove win32/common/libgstgl.def

Just remove win32/common/libgstgl.def

It was always wrong since the symbols being exported in gstgl-1.0.dll
are platform-specific, and the check we do in dist checks it on all
platforms (which usually means Linux) and the list is instead
Linux-specific right now.

Even if we fix that, it can still never be right because it depends on
your configuration even on a specific platform. For instance, when we
start building EGL support on Windows using ANGLE, the symbol list will
change depending on whether that is enabled or not.

We also don't need this anymore since we use GST_EXPORT for all
functions exported on Windows now.