Windows: Remove the workaround using vsnprintf. (#8942)

This is not needed, the functions are both defined in legacy_stdio_definitions.cpp in the same way. So use snprintf directly.

This partly reverts commit b293b01.

But add Link(legacy_stdio_definitions), as that's where these functions reside now, since VS 2015.
https://docs.microsoft.com/en-us/cpp/porting/visual-cpp-change-history-2003-2015?view=vs-2019#stdio_and_conio
(This one is needed regardless of this change)

src/lib_c/x86_64-windows-msvc/c/stdio.cr

@@ -1,14 +1,8 @@
 require "./stddef"
 
+@[Link("legacy_stdio_definitions")]
 lib LibC
   fun printf(format : Char*, ...) : Int
   fun rename(old : Char*, new : Char*) : Int
-  fun vsnprintf(str : Char*, size : SizeT, format : Char*, ap : VaList) : Int
-  fun snprintf = __crystal_snprintf(str : Char*, size : SizeT, format : Char*, ...) : Int
-end
-
-fun __crystal_snprintf(str : LibC::Char*, size : LibC::SizeT, format : LibC::Char*, ...) : LibC::Int
-  VaList.open do |varargs|
-    LibC.vsnprintf(str, size, format, varargs)
-  end
+  fun snprintf(buffer : Char*, count : SizeT, format : Char*, ...) : Int
 end