Remove TODO about checking 256 colors support

After a brief research it seems like there is no reliable portable way
to check that. It all comes down to your terminal just interpreting
the escape codes that the program prints correctly.

img2term.c

@@ -605,9 +605,7 @@ char *shift_args(int *argc, char ***argv)
 
 int main(int argc, char **argv)
 {
-    // TODO: check if the terminal support 256 colors and stuff
-    // And throw an error if it doesn't
-    // (Also, maybe similar 16 colors support??)
+    // TODO: 16 colors support
     assert(argc > 0);
     const char *program = shift_args(&argc, &argv);