unix/modselect: Remove unix-specific implementation of select module.

The unix port has a custom select module which only works with objects that
have a file descriptor, eg files and sockets.  On the other hand, bare
metal ports use the common extmod/modselect.c implementation of the select
module that supports polling of arbitrary objects, as long as those objects
provide a MP_STREAM_POLL in their ioctl implementation (which can be done
in C or Python).

This commit removes the unix-specific code and makes unix use the common
one provided by extmod/modselect.c instead.  All objects with file
descriptors implement MP_STREAM_POLL so they continue to work.
Signed-off-by: Damien George <damien@micropython.org>

ports/unix/Makefile

@@ -201,7 +201,6 @@ SRC_C += \
 	mpthreadport.c \
 	input.c \
 	modmachine.c \
-	modselect.c \
 	alloc.c \
 	fatfs_port.c \
 	mpbthciport.c \

ports/unix/variants/mpconfigvariant_common.h

@@ -106,14 +106,8 @@
 #define MICROPY_PY_CRYPTOLIB           (1)
 #endif
 
-// Use the posix implementation of the "select" module (unless the variant
-// specifically asks for the MicroPython version).
-#ifndef MICROPY_PY_SELECT
-#define MICROPY_PY_SELECT              (0)
-#endif
-#ifndef MICROPY_PY_SELECT_POSIX
-#define MICROPY_PY_SELECT_POSIX        (!MICROPY_PY_SELECT)
-#endif
+// The "select" module is enabled by default, but disable select.select().
+#define MICROPY_PY_SELECT_SELECT       (0)
 
 // Enable the "websocket" module.
 #define MICROPY_PY_WEBSOCKET           (1)