maIconCache is defined as a std::unordered_map
'interning' a OUString to give to it as a key seems completely
superfluous since no-one will ever use it by address, even less
compare the address.

Change-Id: Iab5d106d7d51fcdfbd0a0dfd50ae28d686b9b2b7
Reviewed-on: https://gerrit.libreoffice.org/15692Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>