esp32: Fix multiple definition errors with mp_hal_stdout_tx functions.

It was noticed that the esp32 port didn't build ulab correctly.  The
problem was a multiple defintion of the 'mp_hal_stdout_tx_str' and
'mp_hal_stdout_tx_strn_cooked' functions.

They were defined in stdout_helpers.c but also in the
ports/esp32/mphalport.c.

Fixed by removing stdout_helpers.c from the build.

Signed-off-by: Michael O'Cleirigh <michael.ocleirigh@rivulet.ca>

ports/esp32/main/CMakeLists.txt

@@ -25,7 +25,6 @@ set(MICROPY_SOURCE_LIB
     ${MICROPY_DIR}/lib/oofatfs/ffunicode.c
     ${MICROPY_DIR}/lib/timeutils/timeutils.c
     ${MICROPY_DIR}/lib/utils/interrupt_char.c
-    ${MICROPY_DIR}/lib/utils/stdout_helpers.c
     ${MICROPY_DIR}/lib/utils/sys_stdio_mphal.c
     ${MICROPY_DIR}/lib/utils/pyexec.c
 )