Drop libbz2 dependency

Dynamic linking against libbz2 was problematic on some distributions.
We only needed it for pcre's CLI (which we don't use) and ffmpeg's mkv
decompression in case a user forced encoder to use it, because its usage
is discouraged by the standard.

See https://github.com/itgmania/itgmania/issues/217 for the related
discussion.

StepmaniaCore.cmake

@@ -158,12 +158,6 @@ include(ExternalProject)
 
 find_package(nasm)
 find_package(yasm)
-
-find_package(BZip2)
-if(NOT ${BZIP2_FOUND} AND NOT MSVC)
-  message(FATAL_ERROR "Bzip2 support required.")
-endif()
-
 find_package(Iconv)
 
 find_package(Threads)