[varLib] Don't remap same DeviceTable twice
[varLib] Don't remap same DeviceTable twice

The variation merger reuses matrix rows / entries when aligning kerning
matrices.  Don't remap same DeviceTable twice.  Alternative fix would
have been to not reuse objects but that slows things down.

Fixes https://github.com/fonttools/fonttools/issues/1206

fonttools/fonttools,googlefonts/fonttools