Drop print_hex.c — replaced by hex8() from hex_print.c

print_hex(uint8_t) was an early bare-metal helper doing exactly what
hex8() in the freshly-ported hex_print.c does (two-digit hex via
putchar()).  hex8() is smaller (asm cp/sbc/daa nibble trick, no LUT)
and consistent with the dec8/hex16/dec16/hex32/dec32 family.

  • Replaced print_hex() calls with hex8() in examples/banked and
    examples/bankedbg.
  • Removed libc/stdio/print_hex.c, dropped its prototype from
    sprinter.h and its entry from lib/Makefile.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

examples/banked/bank2.c

@@ -7,7 +7,7 @@ void bank2_func(int x) __banked
     (void)x;
     puts("BANK2: hello from the second bank!");
     puts("BANK2: window 3 phys page = ");
-    print_hex(_io_page_w3);     /* should be BANK2's phys page */
+    hex8(_io_page_w3);     /* should be BANK2's phys page */
     putchar('\n');
     putchar('2');
     putchar('=');