Remove some unused typedefs from gdbserver

I noticed that gdbserver/win32-low.cc has a few typedefs that are not
used.  This patch removes them.

gdbserver/ChangeLog
2021-04-13  Tom Tromey  <tromey@adacore.com>

	* win32-low.cc (winapi_CreateToolhelp32Snapshot)
	(winapi_Module32First, winapi_Module32Next): Remove typedefs.