Revert "py/{mkenv.mk,mkrules.mk}: Append .exe for Windows executable files."

This reverts commit 3289b9b7a7.
The commit broke building on MINGW because the filename became
micropython.exe.exe.  A proper solution to support more Windows build
environments requires more thought and testing.

py/mkenv.mk

@@ -59,13 +59,7 @@ LD += -m32
 endif
 
 MAKE_FROZEN = $(TOP)/tools/make-frozen.py
-# allow mpy-cross (for WSL) and mpy-cross.exe (for cygwin) to coexist
-ifeq ($(OS),Windows_NT)
-MPY_CROSS = $(TOP)/mpy-cross/mpy-cross.exe
-PROG_EXT = .exe
-else
 MPY_CROSS = $(TOP)/mpy-cross/mpy-cross
-endif
 MPY_TOOL = $(TOP)/tools/mpy-tool.py
 
 all:

py/mkrules.mk

@@ -111,7 +111,7 @@ FROZEN_MPY_PY_FILES := $(shell find -L $(FROZEN_MPY_DIR) -type f -name '*.py' |
 FROZEN_MPY_MPY_FILES := $(addprefix $(BUILD)/frozen_mpy/,$(FROZEN_MPY_PY_FILES:.py=.mpy))
 
 # to build .mpy files from .py files
-$(BUILD)/frozen_mpy/%.mpy: $(FROZEN_MPY_DIR)/%.py $(MPY_CROSS)
+$(BUILD)/frozen_mpy/%.mpy: $(FROZEN_MPY_DIR)/%.py $(TOP)/mpy-cross/mpy-cross
 	@$(ECHO) "MPY $<"
 	$(Q)$(MKDIR) -p $(dir $@)
 	$(Q)$(MPY_CROSS) -o $@ -s $(<:$(FROZEN_MPY_DIR)/%=%) $(MPY_CROSS_FLAGS) $<
@@ -133,13 +133,13 @@ $(PROG): $(OBJ)
 # we may want to compile using Thumb, but link with non-Thumb libc.
 	$(Q)$(CC) -o $@ $^ $(LIB) $(LDFLAGS)
 ifndef DEBUG
-	$(Q)$(STRIP) $(STRIPFLAGS_EXTRA) $(PROG)$(PROG_EXT)
+	$(Q)$(STRIP) $(STRIPFLAGS_EXTRA) $(PROG)
 endif
-	$(Q)$(SIZE) $$(find $(BUILD) -path "$(BUILD)/build/frozen*.o") $(PROG)$(PROG_EXT)
+	$(Q)$(SIZE) $$(find $(BUILD) -path "$(BUILD)/build/frozen*.o") $(PROG)
 
 clean: clean-prog
 clean-prog:
-	$(RM) -f $(PROG)$(PROG_EXT)
+	$(RM) -f $(PROG)
 	$(RM) -f $(PROG).map
 
 .PHONY: clean-prog