remove HTMLPurifier cache files; ignore them in the future

.gitignore

...ializer/HTML/3.3.0,100651a4228f7f5e2ed76d7bf716cc0c,1.ser

...ializer/HTML/3.3.0,45b0517676cd053c6e36094a738f227d,1.ser

...ializer/HTML/3.3.0,4a226eae658b18b7585dcf9b89faec84,1.ser

...ializer/HTML/3.3.0,82083055ed354c59f36a803cf20c7427,1.ser

...ializer/HTML/3.3.0,a76f5d9f2e01e9a480f6210288f9e4a9,1.ser

...ializer/HTML/3.3.0,c7c3b486561915de4f65b513847bcd4d,1.ser

...ializer/HTML/3.3.0,f60bb57a6b2f21caca005e22d34a48b9,1.ser