fix(extmarks): fix signs

Don't add sign extmarks to state->active. Instead when drawing signs,
perform a full line scan for sign marks. This allows decor_redraw_line
to be moved back inside the `!number_only` block in screen.c, which
prevents decor scans when redrawing the number column when
'relativenumber' is set.

Fixes: #17638

src/nvim/decoration.h

@@ -80,7 +80,6 @@ typedef struct {
   int col_until;
   int current;
   int eol_col;
-  bool has_sign_decor;
 } DecorState;
 
 typedef struct {