This reverts commit e559c0c9; just turn the
deleted overloads into non-friend functions (and rely on any other overloads to
be still found via ADL).

Change-Id: I2af834162cab2e71ed9e32ae6903bc9f86d77ba2
Reviewed-on: https://gerrit.libreoffice.org/30441Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>