mpy-cross: Remove setting of MICROPY_FORCE_32BIT=1 from Makefile.

Building in 32-bit mode was only to reduce binary size on 64-bit machines
and is otherwise not needed.  Having it forced to 32-bit meant an
unnecessary dependency on 32-bit libraries that is now removed.

mpy-cross/Makefile

@@ -1,5 +1,3 @@
-MICROPY_FORCE_32BIT = 1
-
 include ../py/mkenv.mk
 
 # define main target