Unify icons for OpenPGP and S/MIME

* src/olflange.cpp (install_forms): Remove old forms.
* forms/gpgol-form-signed-smime-opaque.cfg,
forms/gpgol-form-smime.cfg: Remove.
forms/gpgol-*.cfg: Use Outlook style icon.

The removed forms should result in Outlook icons not beeing
replaced.
Changing the other forms should result in using similar
icons then outlook ones. The outlook icons are not exposed
to us, so we use similar ones but they can differ slightly
depending on the Outlook Version and style.

• GnuPG-Bug-Id: T4142