Fix MacVim GUI to hard-code sign icon to width of 2 to reduce dependency

Prevoiusly the code uses strlen to get the sign icon width but it's
always 2 (according to Vim's documentation, see ":h sign-define"), and
the function it calls (`sign_get_text`) has been removed from Vim
already. Just use the hard-coded 2 for simplicity and reduce dependency.

Author: ychin

src/MacVim/gui_macvim.m

@@ -2369,8 +2369,7 @@
     if (!imgName)
         return;
 
-    char_u *txt = sign_get_text(typenr);
-    int txtSize = txt ? strlen((char*)txt) : 2;
+    const int txtSize = 2; // This is specified in the docs, and is expected to always be the case.
 
     [[MMBackend sharedInstance] drawSign:imgName
                                    atRow:row