VS: Remove the '/guard:cf' flag from v143 link flag table

Apply the change from commit db35e3cf (VS: Fix support for '/guard:cf'
linker flag for v142, 2019-01-24, v3.14.0-rc1~74^2~2) to the v143 flag
table.

The entry for `LinkControlFlowGuard` in `v143_Link.json` does not work
when used in a `.vcxproj` file.  Drop our link flag table entries for
this toolset so that the flag will be passed via `AdditionalOptions`.

Also add a test case.
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