VS: Fix support for '/guard:cf' linker flag

Original header commit v3.9.0-rc6~12^2

Although `$(VCTargetsPath)/1033/link.xml` for v140 and v141 toolsets
contains an entry for `LinkControlFlowGuard`, it does not work when used
in a `.vcxproj` file.  Drop our link flag table entries for these
toolsets so that the flag will be passed via `AdditionalOptions`.