Drop pgm_read_*_near and let headers choose (#12301)

- Drop `pgm_read_*_near` and let headers choose.
- Define `USE_EXECUTE_COMMANDS_IMMEDIATE` as a conditional.
- Add `process_subcommands_now` for SRAM-based commands.

Marlin/src/inc/Conditionals_post.h

@@ -1626,9 +1626,7 @@
 // If platform requires early initialization of watchdog to properly boot
 #define EARLY_WATCHDOG (ENABLED(USE_WATCHDOG) && defined(ARDUINO_ARCH_SAM))
 
-#if ENABLED(G29_RETRY_AND_RECOVER)
-  #define USE_EXECUTE_COMMANDS_IMMEDIATE
-#endif
+#define USE_EXECUTE_COMMANDS_IMMEDIATE ENABLED(G29_RETRY_AND_RECOVER)
 
 #if ENABLED(Z_TRIPLE_STEPPER_DRIVERS)
   #define Z_STEPPER_COUNT 3