QDoc: Drop support for .qhp.sha1 files

Since 0f409aca96a6b51271cf86edef948a7981195cf4 (qtbase), QDoc has generated a .qhp.sha1 file. The purpose of this file was to serve as an identifier that something in the .qhp file changed, such that the .qhp file could be used as a search index.

However, until a very recent bugfix, this didn't work in a cross-platform way. Furthermore, there are several types of changes that do not affect the contents of a .qhp file. Finally, the existence of this file is largely unknown and not used anywhere in Qt.