CMake: Fixup BZip2 dependency

1. Remove `find_package(BZip2 REQUIRED)` from Dependencies.cmake
   because it is not used by devilutionx directly (it's a libmpq dependency).
2. For emscripten, use the built-in version of bzip2.
3. Reverts 0b4705cf1c, which added
   a redundant `find_package(ZLIB REQUIRED)` to `Dependencies.cmake`.

CMake/Dependencies.cmake

@@ -7,18 +7,18 @@ if(EMSCRIPTEN)
   emscripten_system_library("zlib" ZLIB::ZLIB USE_ZLIB=1)
 else()
   dependency_options("zlib" DEVILUTIONX_SYSTEM_ZLIB ON DEVILUTIONX_STATIC_ZLIB)
-  if(DEVILUTIONX_SYSTEM_ZLIB)
-    find_package(ZLIB REQUIRED)
-  else()
+  if(NOT DEVILUTIONX_SYSTEM_ZLIB)
     add_subdirectory(3rdParty/zlib)
   endif()
 endif()
 
-dependency_options("bzip2" DEVILUTIONX_SYSTEM_BZIP2 ON DEVILUTIONX_STATIC_BZIP2)
-if(DEVILUTIONX_SYSTEM_BZIP2)
-  find_package(BZip2 REQUIRED)
+if(EMSCRIPTEN)
+  emscripten_system_library("bzip2" BZip2::BZip2 USE_BZIP2=1)
 else()
-  add_subdirectory(3rdParty/bzip2)
+  dependency_options("bzip2" DEVILUTIONX_SYSTEM_BZIP2 ON DEVILUTIONX_STATIC_BZIP2)
+  if(NOT DEVILUTIONX_SYSTEM_BZIP2)
+    add_subdirectory(3rdParty/bzip2)
+  endif()
 endif()
 
 if(EMSCRIPTEN)