DRTVWR-418, MAINT-7038: Don't add double quotes to channel name.

The CMake directive that passes VIEWER_CHANNEL to the C++ compiler as
LL_VIEWER_CHANNEL was enclosing the VIEWER_CHANNEL value in double quotes. At
this point in history, those double quotes literally become part of the
LL_VIEWER_CHANNEL value, causing the viewer to construct a bad Viewer Version
Manager query containing those double quotes. Removing them fixes the query.