Help: Drop incorrect versionadded for try_compile result variable

In commit c705279b (Help: Add `.. versionadded` directives to commands
documentation, 2020-11-08, v3.20.0-rc1~508^2) we accidentally added
``versionadded`` markup suggesting that the first argument to
`try_compile` was fixed as `RESULT_VAR` prior to CMake 3.14.  This was
probably due to misinterpreting the change from commit 7975edea (Help:
User-provided variable names for try_* commands, 2019-02-24,

The result variable has never been fixed.  Drop the incorrect markup.
32 jobs for !6923 with help-try-compile-result-var in 15 minutes and 31 seconds (queued for 4 seconds)
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