remove the unused root first/last from parse ctx

docs/metadesk_reference.md

@@ -432,8 +432,6 @@ main:
 
 @send(Parsing)
 @struct MD_ParseCtx: {
-    first_root: *MD_Node,
-    last_root: *MD_Node,
     first_error: *MD_Error,
     last_error: *MD_Error,
     at: *MD_u8,

source/md.h

@@ -507,8 +507,6 @@ enum
 typedef struct MD_ParseCtx MD_ParseCtx;
 struct MD_ParseCtx
 {
-    MD_Node *first_root;
-    MD_Node *last_root;
     MD_Error *first_error;
     MD_Error *last_error;
     MD_u8 *at;

source/md_impl.c

@@ -1139,7 +1139,6 @@ MD_FUNCTION_IMPL MD_ParseCtx
 MD_Parse_InitializeCtx(MD_String8 filename, MD_String8 contents)
 {
     MD_ParseCtx ctx = MD_ZERO_STRUCT;
-    ctx.first_root = ctx.last_root = MD_NilNode();
     ctx.at = contents.str;
     ctx.file_contents = contents;
     ctx.filename = filename;