Remove HTML entity parsing (#274)

## Summary

* Remove HTML entity parsing
  * This has been completely useless since the introduction of expat
* expat tries to parse all entities in the document, but only knows of
HTML ones
* Parsing will never end with HTML entities in the text, so the
additional step to parse them that we had went completely unused
* We should figure out the best way to parse that content in the future,
but for now remove that module as it generates a lot of heap allocations
with its map and strings

lib/Epub/Epub/htmlEntities.h

@@ -1,7 +0,0 @@
-// from
-// https://github.com/atomic14/diy-esp32-epub-reader/blob/2c2f57fdd7e2a788d14a0bcb26b9e845a47aac42/lib/Epub/RubbishHtmlParser/htmlEntities.cpp
-
-#pragma once
-#include <string>
-
-std::string replaceHtmlEntities(const char* text);