py/py.mk: Remove extra build dir created for frozen_content.

This was originally needed because the .c --> .o rule is:

    $(BUILD)/%.o: %.c

and because the generated frozen_content.c is inside build-FOO, it must
therefore generate build-FOO/build-FOO/frozen_content.o.

But 2eda513870 added a new build rule for
pins.c that can also be used for frozen_content.c.

Signed-off-by: iabdalkader <i.abdalkader@gmail.com>

py/py.mk

@@ -203,7 +203,7 @@ PY_O += $(PY_CORE_O)
 
 # object file for frozen code specified via a manifest
 ifneq ($(FROZEN_MANIFEST),)
-PY_O += $(BUILD)/$(BUILD)/frozen_content.o
+PY_O += $(BUILD)/frozen_content.o
 endif
 
 # Sources that may contain qstrings