VS: Add toolset v142 CSharp flag table
While the flag tables for C and C++ were generated from MSBuild
files, the CSharp flag tables were written by hand. Copy the
flag table to use for the
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)