Makefile: do not automatically set WINDOWSHELL

According to this answer--
https://stackoverflow.com/a/45952425
--Make will always prefer a unix shell, even on Windows,
if one can be found in Path. So we can't check PATH to
determine if it's a Windows shell... this is just too much
bother.

src/Makefile.d/detect.mk

@@ -29,10 +29,6 @@ $(call Print,$(_m))
 
 # go for a 32-bit sdl mingw exe by default
 MINGW:=1
-# cmd.exe uses native Windows semicolon delimited PATH
-ifneq (,$(findstring ;,$(PATH)))
-WINDOWSHELL:=1
-endif
 
 else # if you on the *nix