Autogen: Revert processing of .hh files for compatibility

Since commit 4a9154537c (Autogen: Use cmake::IsHeader/SourceExtension
for file type detection, 2019-07-02, v3.16.0-rc1~470^2~4) we process
`.hh` files with `AUTOMOC`.  However, this change can break existing
projects that do not expect the behavior.  Revert it for now.  It can
be restored later via a policy.

Fixes: #20101
4 files changed