fix(flavor: github): new version of github does not use prefix 'user-content' in headers

New version of GFM does not add a 'user-content-' before generated header ids.
This matches the new spec.

dist/showdown.js

@@ -156,7 +156,6 @@ var showdown = {},
     flavor = {
       github: {
         omitExtraWLInCodeBlocks:              true,
-        prefixHeaderId:                       'user-content-',
         simplifiedAutoLink:                   true,
         excludeTrailingPunctuationFromURLs:   true,
         literalMidWordUnderscores:            true,

src/showdown.js

@@ -11,7 +11,6 @@ var showdown = {},
     flavor = {
       github: {
         omitExtraWLInCodeBlocks:              true,
-        prefixHeaderId:                       'user-content-',
         simplifiedAutoLink:                   true,
         excludeTrailingPunctuationFromURLs:   true,
         literalMidWordUnderscores:            true,