The following steps helped me to remove Microsoft.ReportViewer.Common,Version=12.0.0,PublicKeyToken=89845dcd8080cc91
from gac
cd %systemroot%\assembly\
From here you may have more that one "gac" directory, so you will have to search within each to find your component.
cd gac_msil
cd <assembly DLL name>
cd <assembly version number>__<public key token>
For example: 12.0.0.0__89845dcd8080cc91
erase *.*
Say "y" to "are you sure".
cd ..
rd <assembly version number>__<public key token>
cd ..
rd <assembly DLL name>