Remove state.linkLevel

It's redundant after the changes in links algorithm.

lib/rules_inline/footnote_inline.js

@@ -40,11 +40,9 @@ module.exports = function footnote_inline(state, silent) {
       id: footnoteId,
       level: state.level
     });
-    state.linkLevel++;
     oldLength = state.tokens.length;
     state.parser.tokenize(state);
     state.env.footnotes.list[footnoteId] = { tokens: state.tokens.splice(oldLength) };
-    state.linkLevel--;
   }
 
   state.pos = labelEnd + 1;

lib/rules_inline/links.js

@@ -99,9 +99,6 @@ module.exports = function links(state, silent) {
     //
     if (typeof state.env.references === 'undefined') { return false; }
 
-    // do not allow nested reference links
-    if (state.linkLevel > 0) { return false; }
-
     // [foo]  [bar]
     //      ^^ optional whitespace (can include newlines)
     for (; pos < max; pos++) {
@@ -166,9 +163,7 @@ module.exports = function links(state, silent) {
         title: title,
         level: state.level++
       });
-      state.linkLevel++;
       state.parser.tokenize(state);
-      state.linkLevel--;
       state.push({ type: 'link_close', level: --state.level });
     }
   }

lib/rules_inline/state_inline.js

@@ -20,9 +20,6 @@ function StateInline(src, parserInline, options, env, outTokens) {
 
   // Link parser state vars
 
-  this.linkLevel = 0;     // Increment for each nesting link. Used to prevent
-                          // nesting in definitions
-
   this.linkContent = '';  // Temporary storage for link url
 
   this.labelUnmatchedScopes = 0; // Track unpaired `[` for link labels