Remove minimp3

MP3 emulation already went through FFmpeg, leaving MiniMp3Audio dead.
The one live user was loading MP3 UI sound effects (custom achievement
sounds), which now splits the file into frames and decodes them with
the FFmpeg MP3 decoder.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

libretro/Makefile.common

@@ -25,7 +25,6 @@ INCFLAGS += \
 	-I$(EXTDIR)/glslang \
 	-I$(EXTDIR)/glslang-build \
 	-I$(EXTDIR)/native \
-	-I$(EXTDIR)/minimp3 \
 	-I$(EXTDIR)/zlib \
 	-I$(EXTDIR)/cpu_features/include \
 	-I$(ZSTDDIR) \
@@ -174,9 +173,6 @@ endif
 
 SOURCES_C += $(LIBRETRODIR)/ext/glew/glew.c
 
-SOURCES_CXX += \
-	$(EXTDIR)/minimp3/minimp3.cpp
-
 SOURCES_C += \
 	$(EXTDIR)/libkirk/AES.c \
 	$(EXTDIR)/libkirk/amctrl.c \