libphobos: Don't call __gthread_key_delete in the emutls destroy function.
Fixes a EXC_BAD_ACCESS issue seen on Darwin when the libphobos DSO gets
unloaded. Based on reading libgcc's emutls implementation, as it
doesn't call __gthread_key_delete directly, neither should libphobos.
libphobos/ChangeLog:
* libdruntime/gcc/emutls.d (emutlsDestroyThread): Don't remove entry
from global array.
(_d_emutls_destroy): Don't call __gthread_key_delete.