Don't give Thunderbird its own TMPDIR anymore and drop the corresponding, incomplete AppArmor profile adjustments (refs: #15610)

The rationale provided for this customization (commit:a1fd1f0f, #9558) does not
hold here: the AppArmor profile allows Thunderbird to access /tmp anyway.

Besides, the AppArmor profile tweaks we had in place to match this custom
TMPDIR were incomplete: for example, as reported on #15395#note-24
this broke importing public OpenPGP keys from email attachments.