[Keyboard] niu_mini reduce firmware size (#5648)

* Reducing firmware size by removing audio for keymaps that are not using it.

* Update rules.mk

keyboards/niu_mini/keymaps/planck/rules.mk

@@ -0,0 +1 @@
+

keyboards/niu_mini/rules.mk

@@ -58,7 +58,7 @@ COMMAND_ENABLE = no        # Commands for debug and configuration
 NKRO_ENABLE = no            # Nkey Rollover - if this doesn't work, see here: https://github.com/tmk/tmk_keyboard/wiki/FAQ#nkro-doesnt-work
 BACKLIGHT_ENABLE = yes      # Enable keyboard backlight functionality
 MIDI_ENABLE = no            # MIDI controls
-AUDIO_ENABLE = yes           # Audio output on port C6
+AUDIO_ENABLE = no           # Audio output on port C6
 UNICODE_ENABLE = no         # Unicode
 BLUETOOTH_ENABLE = no       # Enable Bluetooth with the Adafruit EZ-Key HID
 RGBLIGHT_ENABLE = yes        # Enable WS2812 RGB underlight.