Don't lock global mutex when deleting wxThread to avoid deadlocks.

Don't lock global mutex when deleting wxThread to avoid deadlocks.

Calling out the user-defined wxThread dtor while holding gs_mutexDeleteThread
lock is a bad idea as it may result in deadlocks if the dtor deletes another
thread. Only lock the mutex directly before manipulating the data it protects.

Thanks to Neno Ganchev.

Closes #11501.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62781 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775