staging: rtl8188eu: remove function which does nothing

The function rtw_mfree_all_stainfo() is just holding the lock,
traversing the list, and then unlocking. It is not doing anything else.
So removed the function and modified the places from where it was called.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>