opengl: gl_api: remove extension field

The clients wanting to find an extension are now using
vlc_gl_HasExtension, which doesn't prevent from using a Core OpenGL
profile, as opposed to the glGetString(GL_EXTENSIONS) call.

modules/video_output/opengl/gl_api.c

@@ -151,16 +151,6 @@ vlc_gl_api_Init(struct vlc_gl_api *api, vlc_gl_t *gl)
     GET_PROC_ADDR_OPTIONAL(ClientWaitSync);
 #undef GET_PROC_ADDR
 
-    GL_ASSERT_NOERROR(&api->vt);
-
-    api->extensions = (const char *) api->vt.GetString(GL_EXTENSIONS);
-    assert(api->extensions);
-    if (!api->extensions)
-    {
-        msg_Err(gl, "glGetString returned NULL");
-        return VLC_EGENERIC;
-    }
-
     GL_ASSERT_NOERROR(&api->vt);
     GLint version;
     api->vt.GetIntegerv(GL_MAJOR_VERSION, &version);

modules/video_output/opengl/gl_api.h

@@ -38,9 +38,6 @@ struct vlc_gl_api {
     /* True if the current API is OpenGL ES, set by the caller */
     bool is_gles;
 
-    /* Available gl extensions (from GL_EXTENSIONS) */
-    const char *extensions;
-
     /* Non-power-of-2 texture size support */
     bool supports_npot;