py/persistentcode: Remove unicode feature flag from .mpy file.

Prior to this commit, even with unicode disabled .py and .mpy files could
contain unicode characters, eg by entering them directly in a string as
utf-8 encoded.

The only thing the compiler disallowed (with unicode disabled) was using
\uxxxx and \Uxxxxxxxx notation to specify a character within a string with
value >= 0x100; that would give a SyntaxError.

With this change mpy-cross will now accept \u and \U notation to insert a
character with value >= 0x100 into a string (because the -mno-unicode
option is now gone, there's no way to forbid this).  The runtime will
happily work with strings with such characters, just like it already works
with strings with characters that were utf-8 encoded directly.

This change simplifies things because there are no longer any feature
flags in .mpy files, and any bytecode .mpy will now run on any target.

Signed-off-by: Damien George <damien@micropython.org>

py/mpstate.h

@@ -55,7 +55,6 @@ enum {
 #if MICROPY_DYNAMIC_COMPILER
 typedef struct mp_dynamic_compiler_t {
     uint8_t small_int_bits; // must be <= host small_int_bits
-    bool py_builtins_str_unicode;
     uint8_t native_arch;
     uint8_t nlr_buf_num_regs;
 } mp_dynamic_compiler_t;