README: Remove unnecessary note about the mp3 backends.

It was outdated already due to autotools removal, and mp3 isn't the only
supported format with more than one possible backends..

Also removed the outdated autotools related build instruction from there

Author: sezero

README.txt

@@ -23,9 +23,6 @@ This library is under the zlib license, see the file "LICENSE.txt" for details.
 Note:
 Support for software MIDI, MOD, and Opus are not included by default because of the size of the decode libraries, but you can get them by running external/download.sh
 - When building with CMake, you can enable the appropriate SDLMIXER_* options defined in CMakeLists.txt. SDLMIXER_VENDORED allows switching between system and vendored libraries.
-- When building with configure/make, you can build and install them normally and the configure script will detect and use them.
 - When building with Visual Studio, you will need to build the libraries and then add the appropriate LOAD_* preprocessor define to the Visual Studio project.
 - When building with Xcode, you can edit the config at the top of the project to enable them, and you will need to include the appropriate framework in your application.
 - For Android, you can edit the config at the top of Android.mk to enable them.
-
-The default MP3 support is provided using drmp3. SDL_mixer also supports using libmpg123: you can enable it by passing --enable-music-mp3-mpg123 to configure.