syscall: delete asm_windows.s

asm_windows.s contains dummy references of syscall.loadlibrary
and syscall.getprocaddress, to trigger ABI wrapper/alias
generation to get ABI0 symbols for them. The comment says they
are called from assembly in other packages, but I couldn't find
where. They are defined in Go and only referenced in Go.

CL 179862 removed dummy references in the runtime. This CL
is similar, for the syscall package.

Also, with CL 306609, they will have ABI0 definitions anyway.

Change-Id: I5c7b0efb9762e4ad9c94f0beea8d053d8c8b2cd1
Reviewed-on: https://go-review.googlesource.com/c/go/+/306709


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>