perf(column): only build fold/sign column when present in 'statuscolumn'

Problem:    The fold and sign column is built and stored regardless of
            whether the corresponding item is present in 'statuscolumn'.
Solution:   Since the 'statuscolumn' parses itself, we can defer
            building the columns until the corresponding item is
            actually encountered.

src/nvim/fold.h

@@ -9,17 +9,6 @@
 #include "nvim/pos.h"
 #include "nvim/types.h"
 
-// Info used to pass info about a fold from the fold-detection code to the
-// code that displays the foldcolumn.
-typedef struct foldinfo {
-  linenr_T fi_lnum;             // line number where fold starts
-  int fi_level;                 // level of the fold; when this is zero the
-                                // other fields are invalid
-  int fi_low_level;             // lowest fold level that starts in the same
-                                // line
-  linenr_T fi_lines;
-} foldinfo_T;
-
 EXTERN int disable_fold_update INIT(= 0);
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS