VS: Add toolset v142 CSharp flag table
While the flag tables for C and C++ were generated from MSBuild .xml
files, the CSharp flag tables were written by hand. Copy the v141
flag table to use for the v142 toolset.
Remove the special case added by !2856 (merged) that mapped the v142 flag table lookup to v141 since we now have the real v142 table.
Fixes: #19828 (closed)