Emscripten: Allow filesystem support. (#3005)

Filesystem functions perfectly work for Emscripten platform. This changes remove extra changes added by #2734.
You can still disable filesystem functions by defining IMGUI_DISABLE_FILE_FUNCTIONS.

docs/CHANGELOG.txt

@@ -85,6 +85,7 @@ Other Changes:
 - Misc: Added misc/single_file/imgui_single_file.h, We use this to validate compiling all *.cpp
   files in a same compilation unit. Actual users of that technique (also called "Unity builds")
   can generally provide this themselves, so we don't really recommend you use this. [@rokups]
+- CI: Added PVS-Studio static analysis on the continuous-integration server. [@rokups]
 - Backends: GLFW, SDL, Win32, OSX, Allegro: Added support for ImGuiMouseCursor_NotAllowed. [@rokups]
 - Backends: GLFW: Added support for the missing mouse cursors newly added in GLFW 3.4+. [@rokups]
 - Backends: SDL: Wayland: use SDL_GetMouseState (because there is no global mouse state available
@@ -98,7 +99,7 @@ Other Changes:
   the later may be problematic if compiling with recent Windows SDK and you want your app to run
   on Windows 7. You can instead try linking with Xinput9_1_0.lib instead. (#2716)
 - Backends: Glut: Improved FreeGLUT support for MinGW. (#3004) [@podsvirov]
-- CI: Added PVS-Studio static analysis on the continuous-integration server. [@rokups]
+- Backends: Emscripten: Avoid forcefully setting IMGUI_DISABLE_FILE_FUNCTIONS. (#3005) [@podsvirov]
 - Examples: Explicitly adding -DIMGUI_IMPL_OPENGL_LOADER_GL3W to Makefile to match linking
   settings (otherwise if another loader such as Glew is accessible, the OpenGL3 backend might
   automatically use it). (#2919, #2798)

imgui_internal.h

@@ -270,9 +270,6 @@ static inline ImVec4 operator*(const ImVec4& lhs, const ImVec4& rhs)
 #endif
 
 // Helpers: File System
-#if defined(__EMSCRIPTEN__) && !defined(IMGUI_DISABLE_FILE_FUNCTIONS)
-#define IMGUI_DISABLE_FILE_FUNCTIONS
-#endif
 #ifdef IMGUI_DISABLE_FILE_FUNCTIONS
 #define IMGUI_DISABLE_DEFAULT_FILE_FUNCTIONS
 typedef void* ImFileHandle;